Microcrystalline semiconductor film, method for manufacturing the same, and method for manufacturing semiconductor device

ABSTRACT

An embodiment of the present invention is a microcrystalline semiconductor film having a thickness of more than or equal to 70 nm and less than or equal to 100 nm and including a crystal grain partly projecting from a surface of the microcrystalline semiconductor film. The crystal grain has an orientation plane and includes a crystallite having a size of 13 nm or more. Further, the film density of the microcrystalline semiconductor film is higher than or equal to 2.25 g/cm 3  and lower than or equal to 2.35 g/cm 3 , preferably higher than or equal to 2.30 g/cm 3  and lower than or equal to 2.33 g/cm 3 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a microcrystalline semiconductor film,a method for manufacturing the microcrystalline semiconductor film, amethod for manufacturing a semiconductor device including themicrocrystalline semiconductor film, and a display device.

In this specification, a semiconductor device means all types of deviceswhich can function by utilizing semiconductor characteristics, and adisplay device, an electro-optical device, a photoelectric conversiondevice, a semiconductor circuit, and an electronic device are allsemiconductor devices.

2. Description of the Related Art

As one type of field-effect transistor, a thin film transistor whosechannel region is formed in a semiconductor film which is formed over asubstrate having an insulating surface is known. Techniques in whichamorphous silicon, microcrystalline silicon, or polycrystalline siliconis used for the semiconductor Film where the channel region is formed inthe thin film transistor have been disclosed (see Patent Documents 1 to5). A typical application of the thin film transistor is a liquidcrystal television device, in which the thin film transistor ispractically used as a switching transistor in each pixel in a displayscreen.

In addition, a photoelectric conversion device has been developed inwhich a microcrystalline silicon film, which is a film of crystallinesilicon capable of being formed by a plasma CVD method, is used as asemiconductor film having a function of photoelectric conversion (forexample, see Patent Document 6).

[Reference]

[Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    2001-053283-   [Patent Document 2] Japanese Published Patent Application No.    H5-129608-   [Patent Document 3] Japanese Published Patent Application No.    2005-049832-   [Patent Document 4] Japanese Published Patent Application No.    H7-131030-   [Patent Document 5] Japanese Published Patent Application No.    2005-191546-   [Patent Document 6] Japanese Published Patent Application No.    2000-277439

SUMMARY OF THE INVENTION

A thin film transistor whose channel region is formed using an amorphoussilicon film has problems of low field-effect mobility and low on-statecurrent. On the other hand, a thin film transistor whose channel regionis formed using a microcrystalline silicon film has such a problem that,though the field-effect mobility is improved, the off-state current ishigh as compared to the thin film transistor whose channel region isformed using an amorphous silicon film and thus sufficient switchingcharacteristics cannot be obtained.

A thin film transistor whose channel region is formed using apolycrystalline silicon film has a field-effect mobility much higherthan and on-state current higher than those of the above two kinds ofthin film transistors. These features enable this kind of thin filmtransistor to be used not only as a switching transistor in a pixel butalso as an element of a driver circuit that needs high-speed operation.

However, a manufacturing process of the thin film transistor whosechannel region is formed using a polycrystalline silicon film involves acrystallization step for a semiconductor film and has a problem ofhigher manufacturing costs as compared to a manufacturing process of thethin film transistor whose channel region is formed using an amorphoussilicon film. For example, a laser annealing technique necessary in theprocess for forming a polycrystalline silicon film prohibits efficientproduction of large-screen liquid crystal panels because an irradiationarea of the laser beam is small.

The size of a glass substrate for manufacturing display panels has grownas follows: the 3rd generation (550 mm×650 mm), the 3.5th generation(600 mm ×720 mm or 620 mm×750 mm), the 4th generation (680 mm×880 mm or730 mm×920 mm), the 5th generation (1100 mm×1300 mm), the 6th generation(1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8thgeneration (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), andthe 10th generation (2950 mm×3400 mm). The increase in size of glasssubstrate is based on the concept of minimum cost design.

However, a technique has not been established yet with which thin filmtransistors capable of high-speed operation can be manufactured withhigh productivity over a large-sized mother glass substrate such as the10th generation mother glass substrate (2950 mm×3400 mm), which is aproblem in the industry.

An object of an embodiment of the present invention is to provide amicrocrystalline semiconductor film having high crystallinity and amanufacturing method thereof Another object of an embodiment of thepresent invention is to provide a method for manufacturing asemiconductor device having favorable electrical characteristics withhigh productivity.

An embodiment of the present invention is a microcrystallinesemiconductor film having a thickness of more than or equal to 70 nm andless than or equal to 100 nm and including a crystal grain part of whichprojects from a surface of the microcrystalline semiconductor film. Thecrystal grain has an orientation plane and includes a crystallite havinga size of 13 nm or more. Further, the film density of themicrocrystalline semiconductor film is higher than or equal to 2.25g/cm³ and lower than or equal to 2.35 g/cm³, preferably higher than orequal to 2.30 g/cm³ and lower than or equal to 2.33 g/cm³. Note that theabove-mentioned crystal grain is also referred to as a mixed phasegrain. The mixed phase grain includes an amorphous semiconductor regionand a crystallite which is a microcrystal that can be regarded as asingle crystal. In some cases, the mixed phase grain may include a twincrystal. Accordingly, the microcrystalline semiconductor film includesan amorphous semiconductor region. Further, since the microcrystallinesemiconductor film includes mixed phase grains, many crystallites areincluded in the microcrystalline semiconductor film.

The size of the crystallite is the value obtained by substituting thefull width at half maximum of a peak indicating the orientation in thespectrum measured by X-ray diffraction, into the Scherrer Equation, andis the average size of a single crystal having an orientation in themicrocrystalline semiconductor film.

One feature of an embodiment of the present invention is that, afterforming a seed crystal including high-crystallinity mixed phase grainsat a low grain density over an insulating film under a first condition,a first microcrystalline semiconductor film is formed on the seedcrystal under a second condition so as to grow the mixed phase grainsand fill a space between the mixed phase grains, and then amicrocrystalline semiconductor film having high crystallinity is formedon the first microcrystalline semiconductor film under a third conditionwithout increasing the space between the mixed phase grains included inthe first microcrystalline semiconductor film.

The first condition that allows formation of high-crystallinity mixedphase grains at a low density is a condition that a deposition gascontaining silicon or germanium is diluted so that the flow rate ofhydrogen is more than or equal to 50 times and less than or equal to1000 times that of the deposition gas and the pressure in a processchamber is higher than or equal to 67 Pa and lower than or equal to50000 Pa, preferably higher than or equal to 67 Pa and lower than orequal to 13332 Pa. The second condition that allows the mixed phasegrains to grow and the space between the mixed phase grains to be filledis a condition that a deposition gas containing-silicon or germanium isdiluted so that the flow rate of hydrogen is more than or equal to 100times and less than or equal to 2000 times that of the deposition gasand the pressure in a process chamber is higher than or equal to 1333 Paand lower than or equal to 50000 Pa, preferably higher than or equal to1333 Pa and lower than or equal to 13332 Pa.

The third condition that allows formation of a microcrystallinesemiconductor film having high crystallinity without increasing thespace between the mixed phase grains included in the firstmicrocrystalline semiconductor film is a condition that the pressure ina process chamber is higher than or equal to 1333 Pa and lower than orequal to 50000 Pa, preferably higher than or equal to 1333 Pa and lowerthan or equal to 13332 Pa, and a first period in which microcrystallinesemiconductor is deposited and a second period that is longer than thefirst period, in which an amorphous semiconductor region included in themicrocrystalline semiconductor is selectively etched, are performedalternately.

Another embodiment of the present invention is a method in which a seedcrystal which includes a mixed phase grain including an amorphoussemiconductor region and a crystallite which is a microcrystal that canbe regarded as a single crystal is formed by a plasma CVD method under afirst condition, a first microcrystalline semiconductor filth is formedover the seed crystal by a plasma CVD method under a second condition,and a second microcrystalline semiconductor film is formed over thefirst microcrystalline semiconductor film under a third condition. Thefirst condition is a condition that a deposition gas containing siliconor germanium and a gas containing hydrogen are used as source gassupplied to a process chamber, the deposition gas is diluted so that theflow rate of hydrogen is more than or equal to 50 times and less than orequal to 1000 times that of the deposition gas, and the pressure in theprocess chamber is higher than or equal to 67 Pa and lower than or equalto 50000 Pa, preferably higher than or equal to 67 Pa and lower than orequal to 13332 Pa. The second condition is a condition that a depositiongas containing silicon or germanium is diluted so that the flow rate ofhydrogen is more than or equal to 100 times and less than or equal to2000 times that of the deposition gas and the pressure in a processchamber is higher than or equal to 1333 Pa and lower than or equal to50000 Pa, preferably higher than or equal to 1333 Pa and lower than orequal to 13332 Pa. The third condition is a condition that the pressurein a process chamber is higher than or equal to 1333 Pa and lower thanor equal to 50000 Pa, preferably higher than or equal to 1333 Pa andlower than or equal to 13332 Pa, and a first period in whichmicrocrystalline semiconductor is deposited and a second period that islonger than the first period, in which an amorphous semiconductor regionincluded in the microcrystalline semiconductor is selectively etched,are performed alternately.

Note that the seed crystal may be in a state in which a plurality ofmixed phase grains are dispersed or in a state in which a plurality ofmixed phase grains are continuously provided in a state of having a filmshape) in some cases. It is preferable to determine the power forgenerating plasma as appropriate according to the flow rate ratio ofhydrogen to the deposition gas containing silicon or germanium.

Further, in one embodiment of the present invention, a rare gas may beadded to the source gas used in at least one of the first condition, thesecond condition, and the third condition.

According to another embodiment of the present invention, a seed crystalincluding high-crystallinity mixed phase grains at a low grain densityis formed over an insulating film by a plasma CVD method under a firstcondition, a first microcrystalline semiconductor film is formed on theseed crystal by a plasma CVD method under a second condition so as togrow the mixed phase grains and fill a space between the mixed phasegrains, and then a microcrystalline semiconductor film having highcrystallinity is formed on the first microcrystalline semiconductor filmby a plasma CVD method under a third condition without increasing thespace between the mixed phase grains included in the firstmicrocrystalline semiconductor film.

Another embodiment of the present invention is a method, formanufacturing a semiconductor device including a thin film transistorwhose channel region is formed using a microcrystalline semiconductorfilm in which the seed crystal, the first microcrystalline semiconductorfilm, and the second microcrystalline semiconductor film are stacked.

Another embodiment of the present invention is a method formanufacturing a photoelectric conversion device in which themicrocrystalline semiconductor film which the seed crystal, the firstmicrocrystalline semiconductor film, and the second. microcrystallinesemiconductor film are stacked is used as at least one of asemiconductor film having p-type conductivity, a semiconductor filmhaving n-type conductivity, and a semiconductor film having a functionof photoelectric conversion.

According to one embodiment of the present invention, a microcrystallinesemiconductor film having high crystallinity can be formed. Further, asemiconductor device having favorable electrical characteristics can bemanufactured with high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1F are cross-sectional views illustrating a method forforming a microcrystalline semiconductor film according to oneembodiment of the present invention;

FIG. 2 is a diagram showing a method for forming a microcrystallinesemiconductor film according to one embodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating a microcrystallinesemiconductor film according to one embodiment of the present invention;

FIGS. 4A to 4E are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 5A and 5B are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 6A to 6C are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention.

FIGS. 7A to 7D are top views each illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIGS. 8A to 8C are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment of thepresent invention;

FIG. 9 is a cross-sectional view illustrating a method for manufacturinga semiconductor device according to one embodiment of the presentinvention;

FIGS. 10A to 10E are cross-sectional views illustrating one embodimentof a method for manufacturing a photoelectric conversion device;

FIG. 11 is a perspective view illustrating an example of an e-bookreader;

FIG. 12A shows results of Raman spectroscopic analysis ofmicrocrystalline semiconductor films and FIG. 12B shows evaluationresults by an in-plane X-ray diffraction method and evaluation resultsby X-ray reflectometry;

FIGS. 13A and 13B are SEM photographs of a microcrystallinesemiconductor film;

FIGS. 14A and 14B are SEM photographs of a microcrystallinesemiconductor Film;

FIGS. 15A and 15B are TEM photographs of a microcrystallinesemiconductor film;

FIGS. 16A and 16B are TEM photographs of a microcrystallinesemiconductor film; and

FIG. 17 is a SEM. photograph of a microcrystalline semiconductor film.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed with reference to the drawings. Note that the presentinvention is not limited to the following description. It will bereadily appreciated by those skilled in the art that the mode anddetails can be changed in various different ways without departing fromthe spirit and the scope of the present invention. Therefore, thepresent invention should not be construed as being limited to thefollowing description of the embodiments and example. Note that the samereference numerals are commonly used to denote the same components amongdifferent drawings.

(Embodiment 1)

In this embodiment, a method for forming a microcrystallinesemiconductor film in which the crystallinity is improved by reductionof a space between mixed phase grains will be described with referenceto FIGS. 1A to 1F and FIG. 2.

As illustrated in FIG. 1A, an insulating film 55 is formed over asubstrate 51, and a seed crystal 57 is formed over the insulating film.55.

As the substrate 51, a glass substrate, a ceramic substrate, a plasticsubstrate which has heat resistance enough to withstand a processtemperature of this manufacturing process, or the like can be used. Inthe case where the substrate does not need a light-transmittingproperty, a metal substrate, such as a stainless steel substrate,provided with an insulating film on its surface may be used. As theglass substrate, for example, an alkali-free glass substrate of bariumborosilicate glass, aluminoborosilicate glass, aluminosilicate glass, orthe like may be used. Note that there is no limitation on the size ofthe substrate 51. For example, any of glass substrates of the 3rd to10th generations which are often used in the field of the above flatpanel displays can be used.

The insulating film 55 can be formed as a single layer or a stackedlayer using a silicon oxide film, a silicon oxynitride film, a siliconnitride film, a silicon nitride oxide film, an aluminum oxide film, analuminum nitride film, an aluminum oxynitride film, or an aluminumnitride oxide film by a CVD method, a sputtering method, or the like.

Note that here, silicon oxynitride contains more oxygen than nitrogen.In the case where measurements are performed using Rutherfordbackscattering spectrometry

(RBS) and hydrogen forward scattering spectrometry (1-IFS), siliconoxynitride preferably contains oxygen, nitrogen, silicon, and hydrogenat 50 at. % to 70 at. %, 0.5 at. % to 15 at. %, 25 at. % to 35 at. %,and 0.1 at. % to 10 at. %, respectively. Further, silicon nitride oxidecontains more nitrogen than oxygen. In the case where measurements areperformed using RBS and HFS, silicon nitride oxide preferably containsoxygen, nitrogen, silicon, and hydrogen at 5 at. % to 30 at. %, 20 at. %to 55 at. %, 25 at. % to 35 at. %, and 10 at. % to 30 at. %,respectively. Note that percentages, of nitrogen, oxygen, silicon, andhydrogen fall within the ranges given above, where the total number ofatoms contained in the silicon oxynitride or the silicon nitride oxideis defined as 100 at. %.

The seed crystal 57 is formed using microcrystalline semiconductortypified by microcrystalline silicon, microcrystalline silicongermanium, microcrystalline germanium, or the like. The seed crystal 57may be in a state in which a plurality of mixed phase grains isdispersed, in a state in which mixed phase grains are continuouslyprovided and form a film, or in a state in which mixed phase grains andan amorphous semiconductor region are continuously provided and form afilm. Therefore, the seed crystal 57 may have a space 57 b betweenadjacent mixed phase grains 57 a without the mixed phase grains 57 aand/or the amorphous semiconductors being in contact with each other.Further, a feature of the seed crystal 57 is such that the density ofthe mixed phase grains (the existing proportion of the mixed phasegrains in a plane) is low and the crystallinity of the mixed phasegrains is high.

The seed crystal 57 is formed in a process chamber of a plasma CVDapparatus by glow discharge plasma using a mixture of hydrogen and adeposition gas containing silicon or germanium, under a first conditionthat enables mixed phase grains having high crystallinity to be formedat a low density. Alternatively, the seed crystal 57 is formed by glowdischarge plasma using a mixture of hydrogen, a deposition gascontaining silicon or germanium, and a rare gas such as helium, neon,argon, krypton, or xenon. Here, microcrystalline silicon,microcrystalline silicon germanium, microcrystalline germanium, or thelike is deposited as the seed crystal 57 under the first condition thatthe deposition gas is diluted so that the flow rate of hydrogen is morethan or equal to 50 times and less than or equal to 1000 times that ofthe deposition gas containing silicon or germanium, and the pressure inthe process chamber is higher than or equal to 67 Pa and lower than orequal to 50000 Pa (higher than or equal to 0.5 Torr and lower than orequal to 370 Torr), preferably higher than or equal to 67 Pa and lowerthan or equal to 13332 Pa (higher than or equal to 0.5 Torr and lowerthan or equal to 100 Torr). At this time, the deposition temperature ispreferably room temperature to 350° C., further preferably 150° C. to280° C. The distance between an upper electrode and a lower electrode inthe process chamber is set to a distance that enables generation ofplasma. With the first condition, crystal growth is promoted and thecrystallinity of the mixed phase grains 57 a in the seed crystal 57 isimproved. In other words, the size of the crystallites included in themixed phase grains 57 a in the seed crystal 57 is increased. Further,the space 57 b is formed between the adjacent mixed phase grains 57 a,which leads to a low density of the mixed phase grains 57 a. Note thatin the above-description of the flow rate of hydrogen and the flow rateof the deposition gas containing silicon or germanium, the depositiongas containing silicon or germanium means a 100% undiluted depositiongas containing silicon or germanium. Accordingly, in the case where thedeposition gas containing silicon or germanium is diluted, the flow rateof the 100% deposition gas containing silicon or germanium in thediluted gas is calculated and then the flow rate of hydrogen is adjustedbased on the flow rate of the 100% deposition gas.

As typical examples of the deposition gas containing silicon orgermanium, there are SiH₄, Si₂H₆, GeH₄, Ge₂H₆, and the like.

When a rare gas such as helium, neon,. argon, krypton, or xenon is addedto the source gas of the seed crystal 57, the deposition rate of theseed crystal 57 is increased, which can reduce the amount of impuritiesmixed in the seed crystal 57. Accordingly, the crystallinity of the seedcrystal 57 can be improved. By adding a rare gas such as helium, neon,argon, krypton, or xenon to the source gas of the seed crystal 57,stable plasma generation is possible without application of high power.Therefore, plasma damage to the seed crystal 57 can be reduced and thecrystallinity of the seed crystal 57 can be improved.

When the seed crystal 57 is formed, glow discharge plasma is generatedby application of high-frequency power with a frequency of 3 MHz to 30MHz, typically 13.56 MHz or 27.12 MHz in the HF band, or high-frequencypower with a frequency of approximately 30 MHz to 300 MHz in the VHFband, typically 60 MHz. Alternatively, glow discharge plasma isgenerated by application of high-frequency power with a microwave of 1GHz or higher. Note that pulsed oscillation by which high-frequencypower is applied in a pulsed manner or continuous oscillation by whichhigh-frequency power is applied continuously can be employed. Inaddition, by superimposing high-frequency power in the HF band andhigh-frequency power in the VHF band on each other, unevenness of plasmaacross a large-sized substrate is also reduced, so that uniformity infilm thickness and film quality can be increased and the deposition ratecan be increased.

The flow rate of hydrogen is set higher than the flow rate of thedeposition gas containing silicon or germanium as described above,whereby the amorphous semiconductor region in the seed crystal 57 isetched while the seed crystal 57 is deposited; thus, the mixed phasegrains 57 a having high crystallinity can be formed and the space 57 bcan be formed between the adjacent mixed phase grains 57 a. Optimalconditions differ according to an apparatus structure and a chemicalstate of a surface on which a film is to be formed: if the mixed phasegrains 57 a are hardly deposited, the flow rate ratio of hydrogen to thedeposition gas containing silicon or germanium may be reduced or the RFelectric power may be reduced; on the other hand, if the grain densityof the mixed phase grains 57 a is high or an amorphous semiconductorregion is larger than a crystalline semiconductor region, the flow rateratio of hydrogen to the deposition gas containing silicon or germaniummay be increased or the RF electric power may be increased. The state ofdeposition of the seed crystal 57 can he evaluated by scanning electronmicroscopy (SEM) and Raman spectroscopy. By employing the aboveconditions of the flow rate ratio and the pressure in the processchamber, the seed crystal 57 can have favorable crystallinity and have asuitable space between the mixed phase grains 57 a. Thus, the mixedphase grains 57 a are formed while the amorphous semiconductor region inthe seed crystal 57 is etched. Accordingly, crystal growth is promotedand the crystallinity of the mixed phase grains 57 a is improved. Thatis, the size of the crystallites in the mixed phase grains 57 a isincreased. Further, since the amorphous semiconductor region between theadjacent mixed phase grains 57 a is etched, the space 57 b is formedbetween the adjacent mixed phase grains 57 a; accordingly, the mixedphase grains 57 a are formed at a low grain density. Note that when theseed crystal 57 is formed under the first condition of this embodiment,the grain sizes of the mixed phase grains 57 a are uneven in some cases.

Then, as illustrated in FIG. 1B, a first microcrystalline semiconductorfilm 59 is formed on the seed crystal 57. The first microcrystallinesemiconductor film 59 is formed under a condition that enablescrystallites of the seed crystal 57 to grow so that the space betweenthe mixed phase grains is filled.

The first microcrystalline semiconductor film 59 is formed under asecond condition in a process chamber of the plasma CVD apparatus byglow discharge plasma using a mixture of hydrogen and a deposition gascontaining silicon or germanium. Alternatively, the firstmicrocrystalline semiconductor film 59 may be formed by glow dischargeplasma using a mixture of a source gas of the second condition and arare gas such as helium, neon, argon, krypton, or xenon. Here, thesecond condition is a condition that the deposition gas containingsilicon or germanium is diluted so that the flow rate of hydrogen ismore than or equal to 100 times and less than or equal to 2000 timesthat of the deposition gas, and the pressure inside the process chamberis higher than or equal to 1333 Pa and lower than or equal to 50000 Pa(higher than or equal to 10 Torr and lower than or equal to 370 Torr),preferably higher than or equal to 1333 Pa and lower than or equal to13332 Pa (higher than or equal to 10 Torr and lower than or equal to 100Torr).

As the first microcrystalline semiconductor film 59, a microcrystallinesilicon film, a microcrystalline silicon germanium film, amicrocrystalline germanium film, or the like is formed under the abovesecond condition. Therefore, in the first microcrystalline semiconductorfilm 59, the ratio of the crystal regions to the amorphous semiconductorregion is increased and the space between the crystal regions isreduced, whereby the crystallinity is improved. The depositiontemperature at this time is preferably room temperature to 350° C.,further preferably 150° C. to 280° C. The distance between an upperelectrode and a lower electrode in the process chamber is set to adistance that enables generation of plasma.

The condition for generating glow discharge plasma in the formation ofthe seed crystal 57 can be employed as appropriate for the formation ofthe first microcrystalline semiconductor film 59. In the case where thecondition for generating glow discharge plasma in the formation of theseed crystal 57 and that in the formation of the first microcrystallinesemiconductor film 59 are the same, throughput can he increased.However, the conditions may be different from each other.

The first microcrystalline semiconductor film 59 is formed under thesecond condition that enables crystallites of the seed crystal 57 togrow so that the space 57 b between the mixed phase grains 57 a isfilled. Typically, in the second condition, the deposition gascontaining silicon or germanium is diluted so that the flow rate ofhydrogen is more than or equal to 100 times and less than or equal to2000 times that of the deposition gas, and the pressure in the processchamber is higher than or equal to 1333 Pa and lower than or equal to50000 Pa (higher than or equal to 10 Torr and lower than or equal to 370Torr), preferably higher than or equal to 1333 Pa and lower than orequal to 13332 Pa (higher than or equal to 10 Torr and lower than orequal to 100 Torr). Note that in the above-description of the flow rateof hydrogen and the flow rate of the deposition gas containing siliconor germanium, the deposition gas containing silicon or germanium means a100% undiluted deposition gas containing silicon or germanium.Accordingly, in the case where the deposition gas containing silicon orgermanium is diluted, the flow rate of the 100% deposition gascontaining silicon or germanium in the diluted gas is calculated andthen the flow rate of hydrogen is adjusted based on the flow rate of the100% deposition gas. Because of such high pressure in the processchamber in the above-described second condition, the mean free path ofthe deposition gas is short; thus, hydrogen radicals and hydrogen ionslost energy every time they collide with each other. Accordingly, theenergy of hydrogen radicals and hydrogen ions becomes lower, which leadsto improvement of the coverage and reduction of ion damage, contributingto reduction of defects. Further, in the above condition, since thedilution ratio of the deposition gas containing silicon or germanium ishigh and the amount of generated hydrogen radicals is increased, thecrystal grows using the crystallites in the mixed phase grains 57 a as anucleus while the amorphous semiconductor region is etched. As a result,in the first microcrystalline semiconductor film 59, the ratio of thecrystal regions to the amorphous semiconductor region is increased andthe crystallinity is improved. Further, defects caused in the firstmicrocrystalline semiconductor film 59 during the deposition arereduced.

When mixed phase grains of the first microcrystalline semiconductor film59 are newly generated in the space 57 b between the mixed phase grains57 a of the seed crystal 57, the size of the mixed phase grains of thefirst microcrystalline semiconductor film 59 becomes smaller. Therefore,it is preferable that the frequency of generation of the mixed phasegrains of the first microcrystalline semiconductor film 59 be lower thanthat of the mixed phase grains 57 a of the seed crystal 57. In thismanner, using the crystallites included in the mixed phase grains 57 aof the seed crystal 57 as nuclei, crystal growth from the seed crystal57 can be promoted primarily.

Crystal growth of the first microcrystalline semiconductor film 59occurs using the crystallites in the mixed phase grains 57 a of the seedcrystal 57 as nuclei. Further, the size of the mixed phase grain of thefirst microcrystalline semiconductor film 59 depends on the spacebetween the mixed phase grains 57 a of the seed crystal 57. Therefore,when the grain density of the mixed phase grains 57 a of the seedcrystal 57 is low, the space between the mixed phase grains 57 a islarge; thus, the crystal growth distance of the mixed phase grains ofthe first microcrystalline semiconductor film 59 can be increased andthe size of the mixed phase grains in the first microcrystallinesemiconductor film 59 can be increased.

Next, as illustrated in FIG. 1C, a second microcrystalline semiconductorfilm 61 is formed over the first microcrystalline semiconductor film 59.The second microcrystalline semiconductor film 61 is formed under acondition that allows a microcrystalline semiconductor film havinghigher crystallinity than the first microcrystalline semiconductor film59 to be formed without increasing the space between the mixed phasegrains included in the first microcrystalline semiconductor film.

The second microcrystalline semiconductor film 61 is formed under athird condition in a process chamber of the plasma CVD apparatus by glowdischarge plasma using a mixture of hydrogen and a deposition gascontaining silicon or germanium. Alternatively, the secondmicrocrystalline semiconductor film 61 is formed under a third conditionby glow discharge plasma using a mixture of a deposition gas containingsilicon or germanium, hydrogen, and a rare gas such as helium, neon,argon, krypton, or xenon. Here, the third condition is a condition thatthe pressure in the process chamber is higher than or equal to 1333 Paand lower than or equal to 50000 Pa, preferably higher than or equal to1333 Pa and lower than or equal to 13332 Pa, and a first period in whichmicrocrystalline semiconductor is deposited and a second period that islonger than the first period, in which an amorphous semiconductor regionincluded in the microcrystalline semiconductor is selectively etched,are performed alternately.

In order to alternately perform the first period for whichmicrocrystalline semiconductor is deposited and the second period thatis longer than the first period, in which an amorphous semiconductorregion included in the microcrystalline semiconductor is selectivelyetched, the flow rate ratio of the deposition gas containing silicon orgermanium to hydrogen is alternately increased and decreased;specifically, the flow rate of the deposition gas containing silicon orgermanium or the flow rate of hydrogen is increased and decreased. Inthe case where the flow rate ratio of hydrogen to the deposition gascontaining silicon or germanium is low, typically, in the case where theflow rate of hydrogen is more than or equal to 100 times and less thanor equal to 2000 times that of the deposition gas, deposition andcrystal growth of the microcrystalline semiconductor is primarilycaused. On the other hand, in the case where the flow rate ratio ofhydrogen to the deposition gas containing silicon or germanium is high,typically, in the case where the flow rate of the deposition gascontaining silicon or germanium is more than or equal to 0 sccm and lessthan or equal to 0.3 sccm and the flow rate of hydrogen is more than1000 sccm, etching of the amorphous semiconductor region included in themicrocrystalline semiconductor is primarily caused. Here, in the casewhere the flow rate of hydrogen is fixed and the flow rate of thedeposition gas containing silicon or germanium is increased anddecreased, the introduction of the same flow rate of hydrogen as that inthe first period to the process chamber enables the pressure in theprocess chamber to be, constant in the first period and the secondperiod; accordingly, uniformity in the film quality of the secondmicrocrystalline semiconductor film can be increased. In the case wherethe flow rate ratio of hydrogen to the deposition gas containing siliconor germanium is high, the amorphous semiconductor region is etchedprimarily rather than the crystallites in the microcrystallinesemiconductor at a pressure in the process chamber of higher than orequal to 1333 Pa and lower than or equal to 50000 Pa, further preferablyhigher than or equal to 1333 Pa and lower than or equal to 13332 Pa.Note that the above-described flow rate ratio of hydrogen to thedeposition gas containing silicon or germanium means the flow rate ratioof hydrogen to the 100% undiluted deposition gas containing silicon orgermanium. Accordingly, in the case where the deposition gas containingsilicon or germanium is diluted, the flow rate of the 100% depositiongas containing silicon or germanium in the diluted gas is calculated andthen the flow rate of hydrogen is adjusted based on the flow rate of the100% deposition gas.

As the second microcrystalline semiconductor film 61, a microcrystallinesilicon film, a microcrystalline silicon germanium film, amicrocrystalline germanium film, or the like is formed under the abovethird condition. Therefore, in the second microcrystalline semiconductorfilm 61, the space between the mixed phase grains of the firstmicrocrystalline. semiconductor film 59 is not increased and thecrystallinity higher than the first microcrystalline semiconductor filmis obtained. The deposition temperature at this time is preferably roomtemperature to 350° C., further preferably 150° C. to 280° C. Thedistance between an upper electrode and a lower electrode in the processchamber is set to a distance that enables generation of plasma.

A rare gas such as helium, neon, argon, krypton, or xenon may be addedto a source gas of the second microcrystalline semiconductor film 61.

The condition for generating glow discharge plasma in the formation ofthe seed crystal 57 can be employed as appropriate for the formation ofthe second microcrystalline semiconductor film 61. In the case where thecondition. for generating glow discharge plasma in the formation of theseed crystal 57, that in the formation of the first microcrystallinesemiconductor film 59, and that in the formation of the secondmicrocrystalline semiconductor film 61 are the same, throughput can beincreased. However, the conditions may be different from each other.

Here, a method for alternately increasing and decreasing the flow rateratio of the deposition gas containing silicon or germanium to hydrogenwill be described with reference to FIG. 2. FIG. 2 is a timing chartshowing temporal changes of the source gas and the power supplied to anapparatus in the method for forming the second microcrystallinesemiconductor film 61 described in this embodiment. in FIG. 2, a solidline 71 indicates on/off states of power supply of the plasma CVDapparatus, a solid line 73 indicates the flow rate of hydrogen, a solidline 75 indicates the flow rate of the deposition gas containing siliconor germanium (Oak is used in FIG. 2), and a solid line 79 indicates theflow rate of a rare gas (argon is used in FIG. 2).

The deposition gas containing silicon or germanium, which is a sourcegas, and hydrogen are introduced into the process chamber of the plasmaCVD apparatus, and the pressure therein is set at a predetermined level.The temperature of the substrate 51 is set at a predeterminedtemperature. At this time, hydrogen is supplied into the process chamberwith a fixed flow rate (flow rate a in FIG. 2).

Next, high-frequency power supply is turned on, and plasma discharge isperformed. The deposition gas containing silicon or germanium whose flowrate is alternately increased and decreased is supplied into the processchamber. Here, alternately increasing and decreasing the flow rate ratioof the deposition gas containing silicon or germanium to hydrogen iscalled cycle flow. In this embodiment, after the power supply is turnedon, the first period in which the deposition gas containing silicon orgermanium with a flow rate c is supplied for t₁ seconds and the secondperiod in which the deposition gas containing silicon or germanium witha flow rate b (0<b<c) is supplied for t₂ seconds are repeated. Here, onefeature is that the t₂ seconds of the second period is longer than thet₁ seconds of the first period. The flow rate of the deposition gascontaining silicon or germanium in the second period is lower than thatin the first period, so that the flow rate ratio of hydrogen to thedeposition gas containing silicon or germanium in the second period ishigher than that in the first period. In the first period, in the casewhere the flow rate of hydrogen is made more than or equal to 100 timesand less than or equal to 2000 times that of the deposition gas,deposition of microcrystalline semiconductor and crystal growth areprimarily caused by plasma discharge performed later. In the secondperiod, etching of the amorphous semiconductor region is primarilycaused.

Radicals are generated from the deposition gas containing silicon orgermanium along with hydrogen radicals in plasma in the first period andthe second period. When the pressure in the process chamber is sethigher than or equal to 1333 Pa and lower than or equal to 50000 Pa(higher than or equal to 10 Torr and lower than or equal to 370 Torr),further preferably higher than or equal to 1333 Pa and lower than orequal to 13332 Pa (higher than or equal to 10 Torr and lower than orequal to 100 Torr), the mean free path of the deposition gas is shortbecause of such high pressure in the process chamber; thus, hydrogenradicals and hydrogen ions lost energy every time they collide with eachother. Accordingly, the energy of hydrogen radicals and hydrogen ionswhen they reach the first microcrystalline semiconductor film 59 becomeslower.

In the first period in which the flow rate of the deposition gascontaining silicon or germanium is high (the period with the flow rate cin FIG. 2) under the above-described pressure, the number of radicalsgenerated from the deposition gas containing silicon or germanium islarger than that in the second period with the flow rate b, so thatdeposition and crystal growth of microcrystalline semiconductor areprimarily caused rather than the etching on the surface of the firstmicrocrystalline semiconductor film 59. When the pressure in the processchamber is set to the above-described level, the energy of radicals andions becomes lower, which leads to reduction of plasma damage on thesecond microcrystalline semiconductor film 61 during deposition,contributing to reduction of defects.

In the second period in which the flow rate of the deposition gascontaining silicon or germanium is low (the period with the flow rate bin FIG. 2) under the above-described pressure, the amorphoussemiconductor region included in the first microcrystallinesemiconductor film 59 which is formed under the second condition and themicrocrystalline semiconductor deposited over the first microcrystallinesemiconductor film 59 is selectively etched by hydrogen radicalsdissociated in plasma; accordingly, the crystallites are exposed.

By repeating the above-described first period and second period,exposure of the crystallites owing to the primary etching of theamorphous semiconductor region in the second period and the crystalgrowth in which the exposed crystallites are used as nuclei in the firstperiod are alternately performed. Accordingly, the size of thecrystallites in the mixed phase grains is increased, and further thecrystal growth is caused to form crystals having an orientation plane.Since the second period is longer than the first period, the amorphoussemiconductor region in the microcrystalline semiconductor issufficiently etched; thus, the amount of amorphous semiconductorincluded in the second microcrystalline semiconductor film can bereduced. As a result, the second microcrystalline semiconductor filmhaving higher crystallinity than the first microcrystallinesemiconductor film can be formed without increasing the space betweenthe mixed phase grains in the first microcrystalline semiconductor film.Further, defects of the second microcrystalline semiconductor film 61can be reduced.

In the second period, by setting the flow rate of the deposition gascontaining silicon or germanium to a slight flow rate, typically to aflow rate of more than 0 sccm and less than or equal to 0.3 sccm, aslight amount of radicals (typically silyl radicals) generated from thedeposition gas is bonded to dangling bonds of the crystallites that areexposed by etching of the amorphous semiconductor region; thus, highcrystallinity can be obtained through crystal growth. That is, crystalgrowth occurs concurrently with the etching, whereby the crystallinityof the second microcrystalline semiconductor film 61 is increased.

Note that here, after the first period in which the deposition gascontaining silicon or germanium flows with the flow rate c, the secondperiod in which the deposition gas containing silicon or germanium flowswith the flow rate b follows; however, after the second period in whichthe deposition gas containing silicon or germanium flows with the flowrate b, the first period in which the deposition gas containing siliconor germanium flows with the flow rate c may follow. Note that t₁ and hare each preferably several seconds to several tens of seconds. When t₁and t₂ are each several minutes, for example, a microcrystallinesemiconductor film having low crystallinity with a thickness of severalnanometers is formed in t₁, and only a surface of the microcrystallinesemiconductor film is reacted in t₂. Accordingly, it is difficult toincrease the crystallinity in the microcrystalline semiconductor film.

Note that here, each of all the first periods, i.e., each of all periodsin which the deposition gas containing silicon or germanium flows withthe flow rate c, takes t₁ seconds; however; they may take differenttimes. Further, here, each of all the second periods, i.e., each of allperiods in which the deposition gas containing silicon or germaniumflows with the flow rate b (b<c), takes t₂ seconds; however, they maytake different times.

Further, as shown by the solid line 79 in FIG. 2, a rare gas such ashelium, neon, argon, krypton, or xenon is not introduced into the sourcegas. However, as shown by a dashed line 77, such a rare gas may beintroduced into the process chamber. Alternatively, a rare gas whoseflow rate is alternately increased and decreased may be introduced intothe process chamber.

Note that although the flow rate of hydrogen is fixed here, the flowrate of hydrogen may be changed as needed to form the secondmicrocrystalline semiconductor film 61. Alternatively, the flow rate ofthe deposition gas containing silicon or germanium may be fixed and theflow rate of hydrogen may be alternately increased and decreased.

By changing the flow rate of the source gas while the high-frequencypower supply is kept on, the deposition rate of the secondmicrocrystalline semiconductor film 61 can be improved.

Note that after the deposition gas containing silicon or germanium isintroduced into the process chamber with the flow rate c, that is, afterthe first period, the high-frequency power supply may be turned off.Alternatively, after the deposition gas containing silicon or germaniumis introduced into the process chamber with the flow rate b, that is,after the second period, the high-frequency power supply may be turnedoff.

Here, the flow rate h of the deposition gas containing silicon orgermanium has the relation: 0<b<c. However, the flow rate b may be 0.That is, the period in which the deposition gas containing silicon orgermanium is introduced and the period in which the deposition gascontaining silicon or germanium is not introduced may be alternatelyprovided. As shown by a dashed line 72, the power may be increased inthe first period in order to increase the deposition rate, and the powermay be decreased in the second period in order to suppress the etchingof the crystallites included in the microcrystalline semiconductor.

Note that if the pressure in the process chamber is higher than or equalto 1333 Pa and lower than or equal to 50000 Pa (higher than or equal to10 Torr and lower than or equal to 370 Torr), preferably higher than orequal to 1333 Pa and lower than or equal to 13332 Pa (higher than orequal to 10 Torr and lower than or equal to 100 Torr), the pressure inthe second condition may be higher than that in the first condition, thepressure in the first condition may be higher than that in the secondcondition, or alternatively, the pressure in the first condition may beequal to that in the second condition.

Through the above-described process, a microcrystalline semiconductorfilm 62 including the seed crystal 57, the first microcrystallinesemiconductor film 59, and the second microcrystalline semiconductorfilm 61 can be formed.

Here, conceptual diagrams illustrating film formation of themicrocrystalline semiconductor film 62 described in this embodiment areshown in FIGS. 1D to 1F. FIGS. 1D to 1F are enlarged views schematicallyshowing the deposition state in FIGS. 1A to 1C.

As illustrated in FIG. 1D, the deposition step of the seed crystal 57 isa step for dispersing the mixed phase grains 57 a in order to increasethe size of the mixed phase grains included in the microcrystallinesemiconductor film 62 formed later. Therefore, in the seed crystal 57,the mixed phase grains 57 a are deposited with the space 57 b providedtherebetween as illustrated in FIG. 1D.

As illustrated in FIG. 1E, the deposition step of the firstmicrocrystalline semiconductor film 59 is a step for causing crystalgrowth using the mixed phase grains 57 a as nuclei and thereby forming afilm including mixed phase grains with extremely little space providedtherebetween. Thus, crystal growth is caused using the mixed phasegrains 57 a as seeds, so that microcrystalline semiconductors 58 aredeposited.

When the pressure in the process chamber is higher than or equal to 1333Pa and lower than or equal to 50000 Pa (higher than or equal to 10 Torrand lower than or equal to 370 Torr), preferably higher than or equal to1333 Pa and lower than or equal to 13332 Pa (higher than or equal to 10Torr and lower than or equal to 100 Torr), the microcrystallinesemiconductors 58 grow not only in the film thickness direction but alsoin the plane direction. As a result, spaces between the microcrystallinesemiconductors 58 are filled and the microcrystalline semiconductors 58become in contact with each other.

As illustrated in FIG. 1F, the deposition step of the secondmicrocrystalline semiconductor film 61 is a step for depositingmicrocrystalline semiconductors 60 having higher crystallinity over themicrocrystalline semiconductors 58. In the deposition step of the secondmicrocrystalline semiconductor film 61, the step for deposition andcrystal growth of microcrystalline semiconductor and the step forprimarily etching an amorphous semiconductor region included in themicrocrystalline semiconductor and exposing crystallites included in themicrocrystalline semiconductor are performed alternately. Further, sincethe pressure in the process chamber is higher than or equal to 1333 Paand lower than or equal to 50000 Pa (higher than or equal to 10 Torr andlower than or equal to 370 Torr), preferably higher than or equal to1333 Pa and lower than or equal to 13332 Pa (higher than or equal to 10Torr and lower than or equal to 100 Torr), the amorphous semiconductorregion in the microcrystalline semiconductor is primarily etched. Forthese reasons, epitaxial growth is likely to be caused at the time ofdeposition of the microcrystalline semiconductor on the exposedcrystallites. Accordingly, in the deposition step of the secondmicrocrystalline semiconductor film 61, orientation of themicrocrystalline semiconductor becomes higher and the microcrystallinesemiconductors 60 having an orientation plane are deposited.

Through the steps of FIGS. 1D to 1F, the microcrystalline semiconductorfilm 62 including mixed phase grains, which has high crystallinity,extremely little space between the mixed phase grains, and anorientation plane, can be formed. The mixed phase grains included in themicrocrystalline semiconductor film 62 may be the microcrystallinesemiconductor deposited in the seed crystal 57, the firstmicrocrystalline semiconductor film 59, or the second microcrystallinesemiconductor film 61. Alternatively, the mixed phase grains included inthe microcrystalline semiconductor film 62 may be the microcrystallinesemiconductor formed by deposition and crystal growth of two or more ofthe seed crystal 57, the first microcrystalline semiconductor film 59,and the second microcrystalline semiconductor film 61.

Here, the shape of the microcrystalline semiconductor film 62 will bedescribed with reference to FIG. 3.

FIG. 3 is a schematic cross-sectional view of the microcrystallinesemiconductor film 62. The microcrystalline semiconductor film 62 havinga thickness of more than or equal to 70 nm and less than or equal to 100nm includes a mixed phase grain 47 part of which projects from a surfaceof the microcrystalline semiconductor film 62. Although a plurality ofmixed phase grains is mixed in a region 45 where mixed phase grains donot project from the surface, the microcrystalline semiconductor film 62has such a feature that the mixed phase grain 47 having a largerdiameter than the mixed phase grain in the region 45 projects from thesurface. Further, the mixed phase grain 47 projecting from the surfacehas a feature of having an orientation plane in the region where themixed phase grain projects from the surface. Therefore, the mixed phasegrain 47 has an angular region when seen from the above. The angularregion has a straight side in a cross-sectional shape. That is, a mixedphase grain having high orientation ratio, that is, having a largecrystallite is included on the surface side of the microcrystallinesemiconductor film 62. The size of the crystallite at this time is 13 nmor more, further preferably 15 nm or more.

The size of the crystallite is calculated by substituting the width athalf the intensity of a peak indicating a plane orientation in thespectrum measured by X-ray diffraction, that is, the full width at halfmaximum, into Formula 1. Note that the size L of the crystalliteobtained according to Formula 1 is the average size of a crystallitehaving a plane orientation represented by the peak in themicrocrystalline semiconductor film.L=Kλ/(βcosθ)  (Formula 1)

Note that K is the Scherrer constant, λ is a wavelength of x-rays, β isa full width at half maximum, and θ is a diffraction angle.

Thus, when the microcrystalline semiconductor film 62 is observed with atransmission electron microscope, the mixed phase grains 47 that havesubstantially the same contrast are observed on the surface side of themicrocrystalline semiconductor film 62. Note that the crystallite in themixed phase grain 47 may he a twin crystal. Further, the crystallite inthe mixed phase grain 47 may include a defect.

The space between adjacent mixed phase grains is extremely little andthe mixed phase grains are densely packed in the microcrystallinesemiconductor film 62; thus, the film density measured by X-rayreflectometry is higher than or equal to 2.25 g/cm³ and lower than orequal to 2.35 g/cm³, preferably higher than or equal to 2.30 g/cm³ andlower than or equal to 2.33 g/cm³. The film density of single crystalsilicon is 2.33 g/cm³; thus, the microcrystalline semiconductor filmformed in this embodiment can be said to have a high density and littlespace. Note that the measured film density may be higher than the filmdensity of single crystal silicon due to measurement variation.

The thickness of the seed crystal 57 is preferably more than or equal to1 nm and less than or equal to 10 nm. If the thickness of the seedcrystal 57 is more than 10 nm, even when the first microcrystallinesemiconductor film 59 is deposited, it is difficult to fill the spacebetween the mixed phase grains 57 a and to etch the amorphoussemiconductor included in the seed crystal 57, which leads to reductionin the crystallinity of the seed crystal 57 and the firstmicrocrystalline semiconductor film 59. In addition, since the mixedphase grain needs to be formed in the seed crystal 57, the thickness ofthe seed crystal 57 is preferably more than or equal to 1 nm.

The total thickness of the first microcrystalline semiconductor film 59and the second microcrystalline semiconductor film 61 is preferably morethan or equal to 60 nm and less than or equal to 100 nm. When the totalthickness of the first microcrystalline semiconductor film 59 and thesecond microcrystalline semiconductor film 61 is 60 nm or more,variation in electrical characteristics of thin film transistors can bereduced; and when the total thickness of the first microcrystallinesemiconductor film 59 and the second microcrystalline semiconductor film61 is 100 nm or less, throughput can be increased and film peeling dueto stress can be suppressed.

The seed crystal 57, the first microcrystalline semiconductor film 59,and the second microcrystalline semiconductor film 61 include amicrocrystalline semiconductor. Note that a microcrystallinesemiconductor is a semiconductor having an intermediate structurebetween an amorphous structure and a crystalline structure (including asingle crystal structure and a polycrystalline structure). Thus, themicrocrystalline semiconductor includes an amorphous semiconductorregion. A microcrystalline semiconductor is a semiconductor having athird state that is stable in terms of free energy and is a crystallinesemiconductor having short-range order and lattice distortion, in whichcolumnar or needle-like mixed phase grains having a diameter greaterthan or equal to 2 nm and less than or equal to 200 nm, preferablygreater than or equal to 10 nm and less than or equal to 80 nm, furtherpreferably greater than or equal to 20 nun and less than or equal to 50nm grow in a direction normal to the substrate surface. Therefore, thereis a case in which a crystal grain boundary is formed at the interfacebetween the columnar or needle-like mixed phase grains. Note that thediameter of a crystal grain means the maximum diameter of a crystalgrain in a plane parallel to the substrate surface.

The peak of the Raman spectrum of microcrystalline silicon, which is atypical example of a microcrystalline semiconductor, is located in alower wave number side than 520 cm⁻¹ which represents single crystalsilicon. That is, the peak of the Raman spectrum of the microcrystallinesilicon exists between 520 cm⁻¹ which represents single crystal siliconand 480 cm⁻¹ which represents amorphous silicon. In addition,microcrystalline silicon includes hydrogen or halogen at 1 atomic % ormore in order to terminate dangling bonds. Moreover, microcrystallinesemiconductor has increased stability and is preferable when containinga rare gas element such as helium, neon, argon, krypton, or xenon tofurther enhance lattice distortion. Such a description of themicrocrystalline semiconductor is disclosed in, for example, U.S. Pat.No. 4,409,134.

According to this embodiment, a microcrystalline semiconductor filmwhose crystallinity is increased by reduction of a space between mixedphase grains can be formed.

(Embodiment 2)

In this embodiment, a method for manufacturing a thin film transistorformed in a semiconductor device that is one embodiment of the presentinvention will be described with reference to FIGS. 4A to 4E, FIGS. 5Aand 513, FIGS. 6A to 6C, and FIGS. 7A to 7D. Note that an n-channel thinfilm transistor has higher carrier mobility than a p-channel thin filmtransistor. Further, it is preferable that all thin film transistorsformed over the same substrate have the same polarity because the numberof manufacturing steps can be reduced in such a case. In thisembodiment, a method for manufacturing an n-channel thin film transistorwill be described.

Note that the term “on-state current” refers to current which flowsbetween a source electrode and a drain electrode when a thin filmtransistor is on. For example, in the case of an n-channel thin filmtransistor, the on-state current refers to current which flows between asource electrode and a drain electrode when gate voltage is higher thanthe threshold voltage of the transistor.

In addition, the term “off-state current” refers to current which flowsbetween a source electrode and a drain electrode when a thin filmtransistor is off. For example, in the case of an n-channel thin filmtransistor, the off-state current refers to current which flows betweena source electrode and a drain electrode when gate voltage is lower thanthe threshold voltage of the thin film transistor.

As illustrated in FIG. 4A, a gate electrode 103 is formed over asubstrate 101. Then, a gate insulating film 105 which covers the gateelectrode 103 (also referred to as a first gate electrode) is formed. Aseed crystal 107 is formed over the gate insulating film 105.

As the substrate 101, any of the substrates that can be used as thesubstrate 51 described in Embodiment 1 can be used as appropriate.

The gate electrode 103 can be formed as a single layer or a stackedlayer using a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, scandium, or nickel oran alloy material which includes any of these material materials as amain component. Further, a semiconductor typified by polycrystallinesilicon doped with an impurity element such as phosphorus, an Ag—Pd—Cualloy, an Al—Nd alloy, an Al—Ni alloy, or the like may be used.

The following are preferable examples of the two-layer structure of thegate electrode 103: a two-layer structure in which a molybdenum film isprovided over an aluminum film, a two-layer structure in which amolybdenum film is provided over a copper film, a two-layer structure inwhich a titanium nitride film or a tantalum nitride film is providedover a copper film, a two-layer structure in which a titanium nitridefilm and a molybdenum film are stacked, a two-layer structure in which afilm of a copper-magnesium alloy containing oxygen and a copper film arestacked, a two-layer structure in which a film of a copper-manganesealloy containing oxygen and a copper film are stacked, a two-layerstructure in which a copper-manganese alloy film and a copper film arestacked, or the like. As a three-layer structure, it is preferable tostack a tungsten film or a tungsten nitride film, an alloy film ofaluminum and silicon or an alloy film of aluminum and titanium, and atitanium nitride film or a titanium film. By stacking a metal filmserving as a barrier film over a film having low electric resistance,electric resistance of the gate electrode 103 can be low and diffusionof metal elements from the metal thin into the semiconductor film can beprevented.

The gate electrode 103 can be formed in the following manner: aconductive film is formed over the substrate 101 by a sputtering methodor a vacuum evaporation method using any of the above materials; a maskis formed over the conductive film by a photolithography method, aninkjet method, or the like; and the conductive film is etched using themask. Alternatively, the gate electrode 103 can be formed by discharginga conductive nanopaste of silver, gold, copper, or the like over thesubstrate by an inkjet method and baking the conductive nanopaste. Inorder to improve adhesion between the gate electrode 103 and thesubstrate 101, a nitride film of any of the above metal materials may beprovided between the substrate 101 and the gate electrode 103. In thisembodiment, a conductive film is formed over the substrate 101 andetched using a resist mask formed by a photolithography method.

Note that a side surface of the gate electrode 103 preferably has atapered shape in order to prevent an insulating film, a semiconductorfilm, and a wiring formed over the gate electrode 103 in later stepsfrom being cut at a step portion of the gate electrode 103. To form thegate electrode 103 having a tapered shape, etching may be performedwhile the resist mask is made to recede.

In the step of forming the gate electrode 103, a gate wiring (a scanline) and a capacitor wiring can also be formed at the same time. Thescan line refers to a wiring for selecting a pixel, and the capacitorwiring refers to a wiring which is connected to one of electrodes of astorage capacitor in a pixel. However, without limitation thereto, thegate electrode 103 and one of or both the gate wiring and the capacitorwiring may be formed separately.

The gate insulating film 105 (also referred to as a first gateinsulating. film) can be formed using any of the insulating films thatcan be used as the insulating film 55 described in Embodiment 1 asappropriate. By forming the gate insulating film 105 using an oxideinsulating film such as a silicon oxide film or a silicon oxynitridefilm, fluctuation in threshold voltage of the thin film transistor canbe reduced.

The gate insulating film 105 can be formed by a CVD method, a sputteringmethod, or the like. The condition for the deposition of the seedcrystal 57 which is described in Embodiment 1 can be employed asappropriate to generate the glow discharge plasma in the step of formingthe gate insulating film 105 by a CVD method. When the gate insulatingfilm 105 is formed at a high frequency of 1 GHz or more with a microwaveplasma CVD apparatus, the breakdown voltage between the gate electrodeand the drain and source electrodes can be improved, whereby a highlyreliable thin film transistor can be obtained.

Further, by forming a silicon oxide film by a CVD method using anorganosilane gas as the gate insulating film 105, the crystallinity ofthe semiconductor film which is formed later can be increased, wherebythe on-state current and the field-effect mobility of the thin filmtransistor can be increased. Examples of the organosilane gas includesilicon-containing compounds such as tetraethoxysilane (TEOS) (chemicalformula: Si(OC₂H₅)₄), tetramethylsilane (TMS) (chemical formula:Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS),octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS),triethoxysilane (SiH(OC₂H₅)₃), and trisdimethylaminosilane(SiH(N(CH₃)₂)₃).

The seed crystal 107 can be formed under the first condition thatenables the mixed phase grains having high crystallinity to be formed ata low density, in a manner similar to that of the seed crystal 57described in Embodiment 1.

In the case where a rare gas such as helium, neon, argon, krypton, orxenon is added to the source gas of the seed crystal 107, thecrystallinity of the seed crystal 107 can be increased. Accordingly, theon-state current and the field-effect mobility of the thin filmtransistor can be increased.

Then, as illustrated in FIG. 4B, a first microcrystalline semiconductorfilm 109 is formed over the seed crystal 107. In a manner similar tothat of the first microcrystalline semiconductor film 59 described inEmbodiment 1, the first microcrystalline semiconductor film 109 can beformed under the second condition that enables the crystallites in theseed crystal 107 to grow to fill a space between the mixed phase grains.

In the case where a rare gas such as helium, neon, argon, krypton, orxenon is added to the source gas of the first microcrystallinesemiconductor film 109, the crystallinity of the first microcrystallinesemiconductor film 109 can be improved as in the case of the seedcrystal 107. Accordingly, the on-state current and the field-effectmobility oldie thin film transistor can be increased.

Next, as illustrated in FIG. 4C, a second microcrystalline semiconductorfilm 110 is formed over the first microcrystalline semiconductor film109. The second microcrystalline semiconductor film 110 can be formedunder the third condition that allows a microcrystalline semiconductorfilm having high crystallinity to be formed without increasing the spacebetween the mixed phase grains included in the first microcrystallinesemiconductor film 109, in a manner similar to that of the secondmicrocrystalline semiconductor film 61 described in Embodiment 1.

Then, as illustrated in FIG. 4D, a semiconductor film 111 is formed overthe second microcrystalline semiconductor film 110. The semiconductorfilm 111 includes a microcrystalline semiconductor region 111 a and anamorphous semiconductor region 111 b. Then, an impurity semiconductorfilm 113 is formed over the semiconductor film 111. Then, a mask 115 isformed of a resist over the impurity semiconductor film 113.

The semiconductor film 111 including the microcrystalline semiconductorregion 111 a and the amorphous semiconductor region 111 b can be formedunder a condition which causes partial crystal growth using the secondmicrocrystalline semiconductor film 110 as a nucleus (a condition underwhich the crystal growth is suppressed).

The semiconductor film 111 is formed in a process chamber of a plasmaCVD apparatus by glow discharge plasma using a mixture of a depositiongas containing silicon or germanium, hydrogen, and a gas containingnitrogen. Examples of the gas containing nitrogen include ammonia,nitrogen, nitrogen fluoride, nitrogen chloride, chloroamine,fluoroamine, and the like. Glow discharge plasma can be generated as inthe case of the seed crystal 107.

In this case, the flow rate ratio of hydrogen to the deposition gascontaining silicon or germanium is set to a flow rate ratio with which amicrocrystalline semiconductor film is formed as in the case of formingthe seed crystal 107, the first microcrystalline semiconductor film 109,or the second microcrystalline semiconductor film 110, and a gascontaining nitrogen is used as a source gas, whereby crystal growth canbe suppressed as compared to the deposition conditions for the seedcrystal 107, the first microcrystalline semiconductor film 109, and thesecond microcrystalline semiconductor film 110. Specifically, since agas containing nitrogen is included in the source gas, the crystalgrowth is partly suppressed at an early stage of the deposition of thesemiconductor film 111; thus, a conical or pyramidal microcrystallinesemiconductor region grows, and an amorphous semiconductor region isformed. Furthermore, at a middle stage or later stage of the deposition,the crystal growth of the conical or pyramidal microcrystallinesemiconductor region stops, and only an amorphous semiconductor regionis deposited. As a result, in the semiconductor film 111, themicrocrystalline semiconductor region 111 a and the amorphoussemiconductor region 111 b which is formed using a well-orderedsemiconductor film having fewer defects and a steep tail of a level at aband edge in the valence band, can be formed.

Here, a typical example of a condition for forming the semiconductorfilm 111 is a condition in which the flow rate of hydrogen is 10 timesto 2000 times, preferably 10 times to 200 times that of the depositiongas containing silicon or germanium. Note that in a typical example of acondition for forming a normal amorphous semiconductor film, the flowrate of hydrogen is 0 times to 5 times that of the deposition gascontaining silicon or germanium.

By adding a rare gas such as helium, neon, argon, krypton, or xenon intoa source gas oldie semiconductor film 111, the deposition rate can beincreased.

The thickness of the semiconductor film 111 is preferably 50 nm to 350nm, further preferably 120 nm to 250 nm.

FIGS. 5A and 5B are enlarged views of a portion between the gateinsulating film 105 and the impurity semiconductor film 113 illustratedin FIG. 4D.

As illustrated in FIG. 5A, the microcrystalline semiconductor region 111a in the semiconductor film 111 has a projection and a depression; theprojection has a conical or pyramidal shape whose width decreases fromthe gate insulating film 105 side toward the amorphous semiconductorregion 111 b side (a tip of the projection has an acute angle). Notethat the microcrystalline semiconductor region 111 a may have aprojection whose width increases from the gate insulating film 105 sidetoward the amorphous semiconductor region 111 b side (an invertedconical or pyramidal shape).

By setting the thickness of the seed crystal 107, the firstmicrocrystalline semiconductor film 109, the second microcrystallinesemiconductor film 110, and the microcrystalline semiconductor region111 a, that is, the distance from the interface between the gateinsulating film 105 and the seed crystal 107 to the tip of theprojection of the microcrystalline semiconductor region 111 a to greaterthan or equal to 5 nm and less than or equal to 310 nm, the projectionof the microcrystalline semiconductor region 111 a is not in contactwith an impurity semiconductor film that is formed later; accordingly,the off-state current of the thin film transistor can be reduced.

Further, it is preferable that the oxygen concentration in thesemiconductor film 111 which is measured by secondary ion massspectrometry (SIMS) be less than 1×10¹⁸ atoms/cm³, because such anoxygen concentration can increase the crystallinity of themicrocrystalline semiconductor region 111 a. The nitrogen concentrationprofile of the semiconductor film 111 which is measured by secondary ionmass spectrometry has a peak concentration greater than or equal to1×10²⁰ atoms/cm³ and less than or equal to 1×10²¹ atoms/cm³, preferablygreater than or equal to 2×10²⁰ atoms/cm³ and less than or equal to1×10²¹ atoms/cm³.

The amorphous semiconductor region 111 b includes an amorphoussemiconductor containing nitrogen. Nitrogen in the amorphoussemiconductor containing nitrogen may exist, for example, as an NH groupor an NH₂ group. Amorphous silicon is used as an amorphoussemiconductor.

The amorphous semiconductor containing nitrogen is a semiconductorhaving lower energy at an Urbach edge measured by a constantphotocurrent method (CPM) or photoluminescence spectroscopy and asmaller amount of absorption spectra of defective levels as compared toa conventional amorphous semiconductor. In other words, as compared tothe conventional amorphous semiconductor, the amorphous siliconcontaining nitrogen is a well-ordered semiconductor having fewer defectsand a steep tail of a level at a band edge in the valence band. Sincethe amorphous semiconductor containing nitrogen has a steep tail of alevel at a band edge in the valence band, the band gap is wide andtunnel current does not flow easily. Therefore, when the amorphoussemiconductor containing nitrogen is provided between themicrocrystalline semiconductor region 111 a and the impuritysemiconductor film 113, the off-state current of the thin filmtransistor can be reduced. In addition, by providing the amorphoussemiconductor containing nitrogen, the on-state current and thefield-effect mobility can be increased.

Further, a peak of a spectrum of the amorphous semiconductor containingnitrogen obtained by low-temperature photoluminescence spectroscopy isgreater than or equal to 1.31 eV and less than or equal to 1.39 eV. Notethat a peak region of a spectrum of a microcrystalline semiconductor,typically microcrystalline silicon, obtained by low-temperaturephotoluminescence spectroscopy is greater than or equal to 0.98 eV andless than or equal to 1.02 eV, which shows that an amorphoussemiconductor containing nitrogen is different from a microcrystallinesemiconductor.

The microcrystalline semiconductor region 111 a, as well as theamorphous semiconductor region 111 b, may include a NH group or an NH₂group.

Further, as illustrated in FIG. 5B, a semiconductor mixed phase grain111 c whose grain size is greater than or equal to 1 nm and less than orequal to 10 nm, preferably greater than or equal to 1 nm and less thanor equal to 5 nm is included in the amorphous semiconductor region 111b, whereby the on-state current and the filed-effect mobility can befurther increased.

A microcrystalline semiconductor having a projection shape (a conical orpyramidal shape) whose width decreases from the gate insulating film 105side toward the amorphous semiconductor region 111 b side is formed inthe following manner. After a microcrystalline semiconductor film isformed under a condition where a microcrystalline semiconductor isdeposited, crystal growth is caused under a condition where crystalgrowth is partly caused while an amorphous semiconductor is deposited.

Since the microcrystalline semiconductor region 111 a in thesemiconductor film 111 has the conical or pyramidal shape or theinverted conical or pyramidal shape, resistance in the verticaldirection (in the film thickness direction) of when voltage is appliedbetween the source and drain electrodes in an on state, i.e. theresistance of the semiconductor film 111 can be lowered. Further, tunnelcurrent does not easily flow since the amorphous semiconductorcontaining nitrogen which is a well-ordered semiconductor having fewerdefects and a steep tail of a level at a band edge in the valence bandis provided between the microcrystalline semiconductor region 111 a andthe impurity semiconductor film 113. Thus, in the thin film transistordescribed in this embodiment, the on-state current and the field-effectmobility can be increased and the off-state current can be reduced.

Here, the semiconductor film 111 including the microcrystallinesemiconductor region 111 a and the amorphous semiconductor region 111 bis formed using the source gas including the gas containing nitrogen.Alternatively, the semiconductor film 111 including the microcrystallinesemiconductor region 111 a and the amorphous semiconductor region 111 bcan be formed in the following manner: the top surface of themicrocrystalline semiconductor film 109 is exposed to a gas containingnitrogen so that nitrogen is adsorbed to the top surface of themicrocrystalline semiconductor film 109, and then film deposition of thesemiconductor film 111 including the microcrystalline semiconductorregion 111 a and the amorphous semiconductor region 111 b is performedusing hydrogen and a deposition gas containing silicon or germanium as asource gas.

The impurity semiconductor film 113 is formed using amorphous silicon towhich phosphorus is added, microcrystalline silicon to which phosphorusis added, or the like. Alternatively, the impurity semiconductor film113 can have a stacked structure of amorphous silicon to whichphosphorus is added and microcrystalline silicon to which phosphorus isadded. Note that, in the case of forming a p-channel thin filmtransistor as a thin film transistor, the impurity semiconductor film113 is formed using microcrystalline silicon to which boron is added,amorphous silicon to which boron is added, or the like. In the casewhere the semiconductor film 111 forms an ohmic contact with wirings 129a and 129 b which are formed later, the impurity semiconductor film 113is not necessarily formed.

The impurity semiconductor film 113 is formed in a process chamber ofthe plasma CVD apparatus by glow discharge plasma using a mixture ofhydrogen, phosphine (diluted with hydrogen or silane), and a depositiongas containing silicon, whereby amorphous silicon to which phosphorus isadded or microcrystalline silicon to which phosphorus is added isformed. In the ease of manufacturing a p-channel thin film transistor,the impurity semiconductor film 113 may be formed using glow dischargeplasma using diborane instead of phosphine.

Further, in the case where the impurity semiconductor film 113 is formedusing microcrystalline silicon to which phosphorus is added ormicrocrystalline silicon to which boron is added, a microcrystallinesemiconductor film, typically a microcrystalline silicon film, is formedbetween the semiconductor film 111 and the impurity semiconductor film113, so that characteristics of the interface can he improved. As aresult, resistance generated at the interface between the impuritysemiconductor film 113 and the semiconductor film 111 can he reduced.Therefore, the amount of current flowing through the source region, thesemiconductor film, and the drain region of the thin film transistor canbe increased and the on-state current and the field-effect mobility canbe increased.

The mask 115 formed of a resist can be formed by a photolithographystep.

Next, the seed crystal 107, the first microcrystalline semiconductorfilm 109, the second microcrystalline semiconductor film 110, thesemiconductor film 111, and the impurity semiconductor film 113 areselectively etched using the mask 115 formed of the resist. Through thisstep, the seed crystal 107, the first microcrystalline semiconductorfilm 109, the second microcrystalline semiconductor film 110, thesemiconductor film 111, and the impurity semiconductor film 113 aredivided for each element, whereby an island-shaped semiconductor stackedbody 117 and an island-shaped impurity semiconductor film 121 areformed. The semiconductor stacked body 117 includes a microcrystallinesemiconductor region 117 a which includes part of the seed crystal 107,part of the first microcrystalline semiconductor film 109, part of thesecond microcrystalline semiconductor film 110, and part of themicrocrystalline semiconductor region 111 a of the semiconductor film111; and an amorphous semiconductor region 117 b which includes part ofthe amorphous semiconductor region 111 b of the semiconductor film 111.Then, the mask 115 formed of the resist is removed (see FIG. 4E).

Next, a conductive film 127 is formed over the impurity semiconductorfilm 121 (see FIG. 6A). The conductive film 127 can be formed as asingle layer or a stacked layer using any of aluminum, copper, titanium,neodymium, scandium, molybdenum, chromium, tantalum, tungsten, and thelike. An aluminum alloy to which an element for preventing a hillock isadded (e.g., an Al—Nd alloy which can be used for the gate electrode103) may also be used. Crystalline silicon to which an impurity elementwhich serves as a donor is added may be used. A stacked-layer structurein which a film on the side that is in contact with the crystallinesilicon to which an impurity element serving as a donor is added isformed using titanium, tantalum, molybdenum, tungsten, or a nitride ofany of these elements, and a layer of aluminum or an aluminum alloy isformed thereover may also be formed. The conductive film 127 may alsohave a stacked-layer structure where aluminum or an aluminum alloy isprovided and titanium, tantalum, molybdenum, tungsten, or nitride of anyof these elements is provided thereon and thereunder. The conductivefilm 127 is formed by a CVD method, a sputtering method, or a vacuumevaporation method. Alternatively, the conductive film 127 may be formedby discharging a conductive nanopaste of silver, gold, copper, or thelike by a screen printing method, an inkjet method, or the like andbaking the conductive nanopaste.

Then, a mask is formed of a resist by a photolithography step, and theconductive film 127 is etched with the use of the resist mask, wherebythe wirings 129 a and 129 b serving as a source electrode and a drainelectrode are formed (see FIG. 6B).

The etching of the conductive film 127 may be either dry etching or wetetching. Note that one of the wirings 129 a and 129 b serves as a signalline as well as a source electrode or a drain electrode. However,without limitation thereto, a signal line may be provided separatelyfrom the source and drain electrodes.

Then, the impurity semiconductor film 121 and the semiconductor stackedbody 117 are partly etched, whereby a pair of impurity semiconductorfilms 131 a and 131 b serving as a source and drain regions is formed.Also, a semiconductor stacked body 133 including a microcrystallinesemiconductor region 133 a and a pair of amorphous semiconductor regions133 b is formed. At this point, the semiconductor stacked body 117 isetched so that the microcrystalline semiconductor region 133 a isexposed, whereby the semiconductor stacked body 133 having the followingstructure is formed: in regions which are covered with the wirings 129 aand 129 b, the microcrystalline semiconductor region 133 a and theamorphous semiconductor regions 133 b are stacked, and in a region whichis covered with neither the wiring 129 a nor the wiring 129 b andoverlaps with at least the gate electrode, the microcrystallinesemiconductor region 133 a is exposed.

Here, ends of the wirings 129 a and 129 b are aligned with ends of theimpurity semiconductor films 131 a and 131 b. However, the ends of thewirings 129 a and 129 b and the ends of the impurity semiconductor films131 a and 131 b are not necessarily aligned with each other; the ends ofthe wirings 129 a and 129 b may be positioned on the inner side than theends of the impurity semiconductor films 131 a and 131 b in a crosssection.

Next, dry etching may be performed. The dry etching is performed under acondition where the exposed microcrystalline semiconductor region 133 aand the exposed amorphous semiconductor regions 133 b are not damagedand the etching rates of the microcrystalline semiconductor region 133 aand the amorphous semiconductor regions 133 b are low. As an etchinggas, Cl₂, CF₄, N₂, or the like is typically used. There is no particularlimitation on an etching method, and an inductively coupled plasma (ICP)method, a capacitively coupled plasma (CCP) method, an electroncyclotron resonance (ECR) method, a reactive ion etching (RIE) method,or the like can be used.

Then, the surfaces of the microcrystalline semiconductor region 133 aand the amorphous semiconductor regions 133 b are subjected to plasmatreatment typified by water plasma treatment, oxygen plasma treatment,ammonia plasma treatment, nitrogen plasma treatment, plasma treatmentusing a mixed gas of oxygen and hydrogen, or the like.

After that, the mask formed of a resist is removed. The mask formed of aresist may be removed before the dry etching of the impuritysemiconductor film 121 and the semiconductor stacked body 117.

As described above, after the microcrystalline semiconductor region 133a and the amorphous semiconductor regions 133 b are formed, dry etchingis additionally performed under a condition where the microcrystallinesemiconductor region 133 a and the amorphous semiconductor regions 133 bare not damaged, whereby an impurity such as a residue over the exposedmicrocrystalline semiconductor region 133 a and the exposed amorphoussemiconductor regions 133 b can be removed. Further, after the dryetching, water plasma treatment or plasma treatment using a mixed gas ofoxygen and hydrogen is successively performed, whereby a residue of theresist mask can be removed and defects of the microcrystallinesemiconductor region 133 a can be reduced. Further, by the plasmatreatment, a higher insulating property between the source region andthe drain region can be obtained. Thus, in the resulting thin filmtransistors, off-state current can be reduced and a variation inelectrical characteristics can be reduced.

Note that the mask is formed of a resist by a photolithography step overthe conductive film 127, and the conductive film 127 is etched using theresist mask; whereby the wirings 129 a and 129 b serving as the sourceand drain electrodes are formed. Then, the impurity semiconductor film121 is etched, whereby the pair of impurity semiconductor films 131 aand 131 b serving as the source and drain regions is formed. At thistime, part of the semiconductor stacked body 117 is etched in somecases. Then, the semiconductor stacked body 117 may be partly etchedafter the mask formed of a resist is removed to form the semiconductorstacked body 133 including the microcrystalline semiconductor region 133a and the pair of amorphous semiconductor regions 133 b.

Since the microcrystalline semiconductor region 117 a is covered withthe amorphous semiconductor region 117 b in the step of removing themask formed of a resist, the microcrystalline semiconductor region 117 ais prevented from being in contact with a resist stripper and a residueof the resist. Further, since the amorphous semiconductor region 117 bis etched using the wirings 129 a and 129 b to expose themicrocrystalline semiconductor region 133 a after the mask formed of aresist is removed, the amorphous semiconductor region which is incontact with the resist stripper and a residue of the resist is not leftin a back channel. Consequently, leakage current due to the resiststripper and the residue of the resist left in the back channel is notgenerated, which can further reduce the off state current of the thinfilm transistor.

Through the above-described process, a single-gate thin film transistorcan be manufactured. With the structure described in this embodiment; asingle-gate thin film transistor with low off-state current, highon-state current, and high field-effect mobility can be manufacturedwith high productivity.

Then, an insulating film (also referred to as a second gate insulatingfilm) 137 is formed over the semiconductor stacked body 133, theimpurity semiconductor films 131 a and 131 b, and the wirings 129 a and129 b. The insulating film 137 can be formed in a manner similar to thatof the gate insulating film 105.

Then, an opening (not illustrated) is formed in the insulating film 137with the use of a mask which is formed of a resist by a photolithographystep. Then, a back gate electrode (also referred to as a second gateelectrode) 139 is formed over the insulating film 137 (see FIG. 6C).Through the above-described process, a dual-gate thin film transistorcan be manufactured. Although not shown, a pixel electrode connected toone of the wirings 129 a and 129 b can be formed at the same time as theformation of the back gate electrode 139.

The back gate electrode 139 can be formed in a manner similar to that ofthe wirings 129 a and 129 b. The back gate electrode 139 can be formedusing a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide, indium zinc oxide, or indium tin oxideto which silicon oxide is added.

Alternatively, the back gate electrode 139 can be formed using aconductive composition including a conductive high molecule (alsoreferred to as a conductive polymer) having a light-transmittingproperty. The back gate electrode 139 preferably has a sheet resistanceof 10000 Ω/sq. or less and a light transmittance of 70% or greater at awavelength of 550 nm. Further, the resistivity of the conductive highmolecule included in the conductive composition is preferably 0.1 Ω·cmor less.

As the conductive high molecule, a so-called π-electron conjugatedconductive high molecule can be used. For example, polyaniline and aderivative thereof, polypyrrole and a derivative thereof, polythiopheneand a derivative thereof, and a copolymer of two or more of aniline,pyrrole, and thiophene and a derivative thereof can be given.

The back gate electrode 139 can be formed by forming a thin film usingany of the above materials by a CVD method, a sputtering method, or avacuum evaporation method, and then etching the thin film using a maskformed of a resist by a photolithography step. Alternatively; the backgate electrode 139 can be formed by discharging a conductive nanopasteof silver, gold, copper, or the like by a screen printing method, aninkjet method, or the like and baking the conductive nanopaste.

Next, the shape of the back gate electrode is described with referenceto FIGS. 7A to 7D, which are plan views of the thin film transistor.

As illustrated in FIG. 7A, the back gate electrode 139 can be formed inparallel to the gate electrode 103. In this case, potential applied tothe back gate electrode 139 and potential applied to the gate electrode103 can be controlled independently. Thus, the threshold voltage of thethin film transistor can be controlled. Further, regions in whichcarriers flow, that is, channel regions are formed on the gateinsulating film 105 side and on the insulating film 137 side in themicrocrystalline semiconductor region; thus, the on-state current of thethin film transistor can be increased.

As illustrated in FIG. 7B, the back gate electrode 139 can be connectedto the gate electrode 103. That is, the gate electrode 103 and the backgate electrode 139 can be connected through an opening 150 formed in thegate insulating film 105 and the insulating film 137. In this case,potential applied to the back gate electrode 139 and potential appliedto the gate electrode 103 are equal. Therefore, regions in whichcarriers flow in a semiconductor film, that is, channel regions areformed on the gate insulating film 105 side and on the insulating film137 side in the microcrystalline semiconductor region; thus, theon-state current of the thin film transistor can be increased.

Further alternatively, as illustrated in FIG. 7C, a structure in whichthe back gate electrode 139 is not connected to the gate electrode 103and is in a floating state can be employed. In that case, channelregions are formed on the gate insulating film 105 side and on theinsulating film 137 side in the microcrystalline semiconductor regionwithout potential applied to the back gate electrode 139; thus, theon-state current of the thin film transistor can be increased.

Further, as illustrated in FIG. 7D, the back gate electrode 139 mayoverlap with the wirings 129 a and 129 b with the insulating film 137provided therebetween. Although the back gate electrode 139 of FIG. 7Ais used in FIG. 7D, it is possible that the back gate electrode 139 ofFIG. 7B or FIG. 7C overlap with the wirings 129 a and 129 b.

In each of the single-gate thin film transistor and the dual-gate thinfilm transistor which are described in this embodiment, a channel regioncan be formed using a microcrystalline semiconductor film whosecrystallinity is increased by reduction of a space between mixed phasegrains. Therefore, the number of carriers that move in the single-gatethin film transistor and dual-gate thin film transistor is increased, sothat the on-state current and the field-effect mobility can beincreased. Further, since the microcrystalline semiconductor film havinghigh crystallinity not only on the first gate insulating film side butalso on the second gate insulating film side is used as the channelregion, the number of carriers that move in the dual-gate thin filmtransistor is increased, so that the on-state current and thefield-effect mobility can be increased. Furthermore, since the amorphoussemiconductor regions 133 b are provided between the microcrystallinesemiconductor region 133 a and the impurity semiconductor films 131 aand 131 b, the off-state current of the thin film transistor can bereduced. Accordingly, the area of the single-gate thin film transistorand the area of the dual-gate thin film transistor can be reduced, whichenables high integration of a semiconductor device. Further, when thethin film transistor described in this embodiment is used for a drivercircuit of a display device, the area of the driver circuit can bedecreased, which enables the frame of the display device to be narrowed.

(Embodiment 3)

In this embodiment, a method for manufacturing a thin film transistor,by which the off-state current can be further reduced as compared to theoff-state current in Embodiment 2, will be described with reference toFIGS. 4A to 4C and FIGS. 8A to 8C.

As in Embodiment 2, the semiconductor stacked body 117 in FIG. 8A isformed through the process illustrated in FIGS. 4A to 4C.

Next, plasma treatment is performed in which a side surface of thesemiconductor stacked body 117 is exposed to plasma 123 with the mask115 formed of a resist left. Here, plasma is generated in an oxidizinggas atmosphere or a nitriding gas atmosphere, and the semiconductor.stacked body 117 is exposed to the plasma 123. Examples of the oxidizinggas include oxygen, ozone, dinitrogen monoxide, water vapor, and a mixedgas of oxygen and hydrogen. Examples of the nitriding gas includenitrogen, ammonia, nitrogen fluoride, nitrogen chloride, chloroamine,and fluoroamine. By generating plasma in an oxidizing gas or a nitridinggas, a radical is generated. The radical reacts with the semiconductorstacked body 117, so that an insulating region that is an oxide ornitride is formed on the side surface of the semiconductor stacked body117. Note that instead of irradiation with plasma, irradiation withultraviolet light may be performed for generation of a radical.

In the case of using oxygen, ozone, water vapor, or a mixed gas ofoxygen and hydrogen as the oxidizing gas, the resist recedes by plasmairradiation, whereby a mask 115 a having a smaller bottom surface isformed as illustrated in FIG. 8B. Consequently, through the plasmatreatment, the exposed impurity semiconductor film 121 is oxidized inaddition to the side surface of the semiconductor stacked body 117,whereby an insulating region 125 that is an oxide or nitride is formedon the side surface of the semiconductor stacked body 117 and on theside surface and part of the top surface of the impurity semiconductorfilm 121.

Next, as described in Embodiment 2, through the process illustrated inFIGS. 6A and 6B, the wirings 129 a and 129 b serving as a sourceelectrode and a drain electrode, the pair of impurity semiconductorfilms 131 a and 131 b serving as a source region and a drain region, thesemiconductor stacked body 133 including the microcrystallinesemiconductor region 133 a and the pair of amorphous semiconductorregions 133 b, and the insulating film 137 are formed as illustrated inFIG. 8C. Thus, a single-gate thin film transistor can be manufactured.

When a back gate electrode is formed over the insulating film 137, adual-gate thin film transistor can be manufactured.

In the single-gate thin film transistor and the dual-gate thin filmtransistor which are described in this embodiment, the channel regioncan be formed using a microcrystalline semiconductor film whosecrystallinity is increased by reduction of a space between mixed phasegrains. Furthermore, by providing the insulating region, which is anoxide or nitride, between the semiconductor stacked body 133 and thewiring 129 a or 129 b, holes injected from the wiring 129 a or 129 b tothe semiconductor stacked body 133 can be reduced; thus, the off-statecurrent of the thin film transistor is reduced and the on-state currentand the field-effect mobility of the thin film transistor are increased.Accordingly, the area of the thin film transistor can be reduced, whichenables high integration of a semiconductor device. Further, when thethin film transistor described in this embodiment is used for a drivercircuit of a display device, the size of the driver circuit can bedecreased, which enables the frame of the display device to be narrowed.

In this embodiment, the description is made with reference to Embodimenthowever, the description can be made with reference to anotherembodiment as appropriate.

(Embodiment 4)

In this embodiment, a method for manufacturing a thin film transistorformed in a semiconductor device that is one embodiment of the presentinvention will be described with reference to FIGS. 4A to 4D, FIGS. 6Ato 6C, and FIG. 9. FIG. 9 shows a step corresponding to the stepillustrated in FIG. 6B.

As in Embodiment 2, the conductive film 127 is formed through theprocess of FIGS. 4A to 4D and FIG. 6A.

Then, as illustrated in FIG. 9, the wirings 129 a and 129 b are formedand the impurity semiconductor film 121 and the semiconductor stackedbody 117 are partly etched, whereby the pair of impurity semiconductorfilms 131 a and 131 b serving as a source and drain regions is formed asin Embodiment 2. Also, a semiconductor stacked body 143 including amicrocrystalline semiconductor region 143 a and an amorphoussemiconductor region 143 b is formed. At this point, part of theimpurity semiconductor film and part of the amorphous semiconductorregion are etched, whereby the semiconductor stacked body 143 having thefollowing structure is formed: in the regions which are covered with thewirings 129 a and 129 b, the microcrystalline semiconductor region 143 aand the amorphous semiconductor region 143 b are stacked, and in theregion which is covered with neither the wiring 129 a nor the wiring 129b and overlaps with the gate electrode, the microcrystallinesemiconductor region 143 a is not exposed and the amorphoussemiconductor region 143 b is exposed. Note that the etching amount ofthe semiconductor stacked body 117 here is smaller than that in the caseillustrated in FIG. 6B.

The subsequent steps are similar to those in Embodiment 2.

Through the above-described process, a single-gate thin film transistorcan be manufactured. Since the back channel side is amorphous in thisthin film transistor, the off-state current can be reduced as comparedto the thin film transistor illustrated in FIG. 6B.

In this embodiment, after the step illustrated in FIG. 9, the back gateelectrode 139 may be formed over the thin film transistor with theinsulating film 137 interposed therebetween as in the step illustratedin FIG. 6C.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

(Embodiment 5)

Thin film transistors are manufactured, and a semiconductor devicehaving a display function (also referred to as a display device) can bemanufactured using the thin film transistors in a pixel portion and alsoin a driver circuit. Further, part or the whole of the driver circuitwhich includes thin film transistors can be formed over the samesubstrate as the pixel portion, whereby a system-on-panel can be formed.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled by acurrent or a voltage, and specifically includes an inorganic EL(electroluminescence) element, an organic EL element, and the like.Furthermore, the display device may include a display medium whosecontrast is changed by an electric effect, such as electronic ink.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel. Further, an embodiment of thepresent invention relates to one mode of an element substrate before thedisplay element is completed in a process of manufacturing the displaydevice, and the element substrate is provided with a plurality of pixelseach having a means for supplying current to the display element.Specifically, the element substrate may he in a state where only a pixelelectrode of the display element is formed, a state in which aconductive film to be a pixel electrode is formed but is not etched yetto form the pixel electrode, or any of the other states.

Note that a display device in this specification refers to an imagedisplay device, a display device, or a light source (including alighting device). Further, the display device also includes any of thefollowing modules in its category: a module to which a connector such asa flexible printed circuit (FPC), a tape automated bonding (TAB) tape,or a tape carrier package (TCP) is attached; a module having a TAB tapeor TCP provided with a printed wiring board at the end thereof; and amodule having an integrated circuit (IC) that is directly mounted on adisplay element by a chip on glass (COG) method.

(Embodiment 6)

In this embodiment, a photoelectric conversion device that is oneembodiment of a semiconductor device will be described. In thephotoelectric conversion device described in this embodiment, amicrocrystalline semiconductor film, as described in Embodiment 1, whosecrystallinity is increased by reduction of the space between mixed phasegrains, is used as a semiconductor film. The microcrystallinesemiconductor film whose crystallinity is increased by reduction of thespace between mixed phase grains is applicable to a semiconductor filmhaving a function of photoelectric conversion, a semiconductor filmhaving a conductivity type, or the like, and is preferably, inparticular, applied to the semiconductor film having a function ofphotoelectric conversion. Further, the microcrystalline semiconductorfilm having high crystallinity and high uniformity of grain sizes ofmixed phase grains can be provided at an interface between thesemiconductor film having a function of photoelectric conversion or thesemiconductor film having a conductivity type and another film. Further,the microcrystalline semiconductor film whose crystallinity is increasedby reduction of the space between mixed phase grains can be provided atan interface between the semiconductor film having a function ofphotoelectric conversion or the semiconductor film having a conductivitytype and another film.

By employing the structure described above, resistance (seriesresistance) caused by the semiconductor film having a function ofphotoelectric conversion or the semiconductor film having a conductivitytype can be reduced, resulting in improvement of characteristics.Further, it is possible to reduce optical and electrical loss at theinterface between the semiconductor film having a function ofphotoelectric conversion or the semiconductor film having a conductivitytype and another film, which can improve the photoelectric conversionefficiency. With reference to FIGS. 10A to 10E, one embodiment of amethod for manufacturing a photoelectric conversion device will bedescribed.

As illustrated in FIG. 10A, a first electrode 202 is formed over asubstrate 200.

As the substrate 200, any of the substrates that can be used for thesubstrate 51 described in Embodiment 1 can be used as appropriate.Alternatively, a plastic substrate can be used. As the plasticsubstrate, it is preferable to use a substrate containing athermosetting resin such an epoxy resin, an unsaturated polyester resin,a polyimide resin, a bismaleimide-triazine resin, or a cyanate resin, ora substrate containing a thermoplastic resin such as a polyphenyleneoxide resin, a polyetherimide resin, or a fluorine resin.

Note that the substrate 200 may have a texture structure. Accordingly,photoelectric conversion efficiency can be improved.

In this embodiment, light is incident on the back side (the lower sidein the drawing) of the substrate 200; thus, a light-transmittingsubstrate is used. Note that when a structure is employed in which lightis incident on a side of a second electrode 210 to be formed later (theupper side in the drawing), the substrate is not limited to alight-transmitting substrate. In this case, a semiconductor substratecontaining a material such as silicon or a conductive substratecontaining a metal material or the like may be used.

The first electrode 202 can be formed using a light-transmittingconductive material that can be used for the back gate electrode 139described in Embodiment 2. The first electrode 202 is formed by asputtering method, a CVD method, a vacuum evaporation method, a coatingmethod, a printing method, or the like.

The first electrode 202 is formed to a thickness of 10 nm to 500 nm,preferably 50 nm to 100 nm. The sheet resistance of the first electrode202 is set to about 20 Ω/sq. to 200 Ω/sq.

Note that in this embodiment, light is incident on the back side (thelower side in the drawing) of the substrate 200; thus, the firstelectrode 202 is formed using a light-transmitting conductive material.Note that when a structure is employed in which light is incident on aside of a second electrode 210 to be formed later (the upper side in thedrawing), the material of the substrate is not limited to alight-transmitting conductive material. In such a case, the firstelectrode 202 can be formed using a conductive material that does nothave a light-transmitting property such as aluminum, platinum, gold,silver, copper, titanium, tantalum, or tungsten. In particular, when amaterial that easily reflects light, such as aluminum, silver, titanium,or tantalum, is used, photoelectric conversion efficiency can besufficiently improved.

Like the substrate 200, the first electrode 202 may have a texturestructure. Further, an auxiliary electrode formed using a low-resistanceconductive material may be separately formed so as to be in contact withthe first electrode 202.

Next, as illustrated in FIG. 10B, a semiconductor film 204 having thefirst conductivity type is formed over the first electrode 202. Thesemiconductor film 204 having the first conductivity type is typicallyformed using a semiconductor film containing a semiconductor material towhich an impurity element imparting a conductivity type is added.Silicon is suitable as a semiconductor material, in terms ofproductivity, price, and the like. When silicon is used as thesemiconductor material, phosphorus or arsenic, which imparts n-typeconductivity, or boron or aluminum, which imparts p-type conductivity,or the like is used as the impurity element imparting a conductivitytype.

In this embodiment, light is incident on the back side (the lower sidein the drawing) of the substrate 200; thus, the conductivity type (thefirst conductivity type) of the semiconductor film 204 having the firstconductivity type is preferably p-type. This is because, for instance,the diffusion length of holes is short owing to the lifetime of a holewhich is as short as about half the lifetime of an electron, and becausemore electrons and holes are formed on the side where light is incidenton a semiconductor film 206 having a function of photoelectricconversion. When the first conductivity type is p-type, current can beextracted before holes are annihilated, whereby a decrease inphotoelectric conversion efficiency can be suppressed. Note that whenthe above problems do not occur, for example, when the semiconductorfilm 206 having a function of photoelectric conversion is sufficientlythin, the first conductivity type may be n-type.

There are other semiconductor materials which can be used for thesemiconductor film 204 having the first conductivity type; for example,silicon carbide, germanium, gallium arsenide, indium phosphide, zincselenide, gallium nitride, and silicon germanium are given. Further, asemiconductor material containing an organic material, a semiconductormaterial containing a metal oxide, or the like can be used. The materialcan be selected as appropriate in consideration of the semiconductorfilm 206 having a function of photoelectric conversion.

Although there is no particular limitation on the crystallinity of thesemiconductor film 204 having the first conductivity type, it ispreferable to use the microcrystalline semiconductor film whosecrystallinity is increased by reduction of the space between mixed phasegrains, which is described in Embodiment 1, as the semiconductor film204 having the first conductivity type. This is because in this case, ascompared with the case of using a conventional microcrystallinesemiconductor film, it is possible to reduce series resistance and tosuppress optical and electrical loss at the interface between themicrocrystalline semiconductor film whose crystallinity is increased andanother film. It is needless to say that another semiconductor such asan amorphous semiconductor, a polycrystalline semiconductor, and asingle crystal semiconductor can also be used.

Like the substrate 200, the semiconductor film 204 having the firstconductivity type may have a texture structure.

The semiconductor film 204 having the first conductivity type can beformed by a plasma CVD method using diborane and a deposition gascontaining silicon. Further, the semiconductor film 204 having the firstconductivity type is formed to a thickness of 1 nm to 100 nm, preferably5 nm to 50 nm.

Alternatively, the semiconductor film 204 having the first conductivitytype may be formed as follows: a silicon film to which an impurityelement imparting a conductivity type is not added is formed by a plasmaCVD method or the like; and boron is added by an ion implantation methodor the like.

Next, as illustrated in FIG. 10C, the semiconductor film 206 having afunction of photoelectric conversion is formed over the semiconductorfilm 204 having the first conductivity type. For the semiconductor film206 having a function of photoelectric conversion, a semiconductor filmformed using a semiconductor material which is similar to that of thesemiconductor film 204 is used. In other words, as the semiconductormaterial, silicon, silicon carbide, germanium, gallium arsenide, indiumphosphide, zinc selenide, gallium nitride, silicon germanium, or thelike is used. In particular, silicon is preferably used. Alternatively,a semiconductor material containing an organic material, a semiconductormaterial containing a metal oxide, or the like can be used.

As the semiconductor film 206 having a function of photoelectricconversion, the microcrystalline semiconductor film whose crystallinityis increased by reduction of the space between mixed phase gains, suchas that described in Embodiment 1, is preferably used. By applying themicrocrystalline semiconductor filth whose crystallinity is increased byreduction of the space between mixed phase grains, such as thatdescribed in Embodiment 1, to the semiconductor film, it is possible toreduce series resistance and to suppress optical and electrical loss atthe interface between the microcrystalline semiconductor film whosecrystallinity is increased and another film as compared with the case ofusing a conventional microcrystalline semiconductor film.

The semiconductor film 206 having a function of photoelectric conversionneeds to absorb light sufficiently, and thus preferably has a thicknessof about 100 nm to 10 μm.

Next, as illustrated in FIG. 10D, a semiconductor film 208 having thesecond conductivity type is formed over the semiconductor film 206having a function of photoelectric conversion. In this embodiment, thesecond conductivity type is n-type. The semiconductor film 208 havingthe second conductivity type can be formed using a material such assilicon to which phosphorus is added as an impurity element imparting aconductivity type. Semiconductor materials that can be used for thesemiconductor film 208 having the second conductivity type are similarto those for the semiconductor film 204 having the first conductivitytype.

The semiconductor film 208 having the second conductivity type can beformed in a manner similar to that of the semiconductor film 204 havingthe first conductivity type. For example, the semiconductor film 208having the second conductivity type can be formed by a plasma CVD methodusing phosphine and a deposition gas containing silicon. As thesemiconductor film 208 having the second conductivity type, themicrocrystalline semiconductor film whose crystallinity is increased byreduction of the space between mixed phase grains, which is described inEmbodiment 1, is preferably used.

In this embodiment, light is incident on the back side (the lower sidein the drawing) of the substrate 200; thus, the conductivity type of thesemiconductor film 208 having the second conductivity type is n-type;however, one embodiment of the disclosed invention is not limitedthereto. When the semiconductor film 204 having the first conductivitytype is n-type, the semiconductor film 208 having the secondconductivity type is p-type.

Then, as illustrated in FIG. 10E, the Second electrode 210 is formedover the semiconductor film 208 having the second conductivity type. Thesecond electrode 210 is formed using a conductive material such asmetal. They can be formed using a material that easily reflects light,such as aluminum, silver, titanium, or tantalum. Such a material ispreferably used because light that fails to be absorbed by thesemiconductor film 206 can be incident on the semiconductor film 206again; thus, photoelectric conversion efficiency can be improved.

As a method of forming the second electrode 210, there are a sputteringmethod, a vacuum evaporation method, a CVD method, a coating method, aprinting method, and the like. Further, the second electrode 210 isformed to a thickness of 10 nm to 500 nm, preferably 50 nm to 100 nm.

Note that in this embodiment, light is incident on the back side (thelower side in the drawing) of the substrate 200 and the second electrode210 is formed using a conductive material which does not have alight-transmitting property, but the structure of the second electrode210 is not limited thereto. For example, when light is incident on thesecond electrode 210 side (the upper side in the drawing), the secondelectrode 210 can be formed using any of the light-transmittingconductive materials for the first electrode 202.

Further, an auxiliary electrode formed using a low-resistance conductivematerial may be formed so as to be in contact with the second electrode210.

By the above method, it is possible to manufacture a photoelectricconversion device in which a microcrystalline semiconductor film whosecrystallinity is increased by reduction of the space between the mixedphase grains is used as any of a semiconductor film having a function ofphotoelectric conversion, a semiconductor film having the firstconductivity type, and a semiconductor film having the secondconductivity type. This can enhance the photoelectric conversionefficiency of a photoelectric conversion device. Note that, as long asthe microcrystalline semiconductor film whose crystallinity is increasedby reduction of the space between the mixed phase grains is used as oneof the semiconductor film having a function of photoelectric conversion,the semiconductor film having the first conductivity type, and thesemiconductor film having the second conductivity type, the film towhich the microcrystalline semiconductor film is applied can be changedas appropriate. Further, when the microcrystalline semiconductor filmswhose crystallinity is increased by reduction of the space between themixed phase grains are used as more than one of the above semiconductorfilms, the photoelectric conversion efficiency can be more effectivelyimproved.

Note that although a photoelectric conversion device having one unitcell is described in this embodiment, a photoelectric conversion devicein which two or more unit cells are stacked as appropriate may beprovided.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

(Embodiment 7)

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofthe electronic devices include television sets (also referred to astelevisions or television receivers), monitors of computers or the like,cameras such as digital cameras or digital video cameras, digital photoframes, cellular phones (also referred to as mobile phones or cellularphone sets), portable game consoles, portable information terminals,audio reproducing devices, large-sized game machines such as pachinkomachines, electronic paper, and the like. The electronic paper can beused for electronic devices for displaying information in a variety offields. For example, an electronic paper can be applied to electronicbook readers (e-book readers), posters, digital signage, publicinformation displays (PIDs), advertisements in vehicles such as trains,and displays of various cards such as credit cards. FIG. 11 illustratesan example of the electronic devices.

FIG. 11 illustrates an example of an electronic book reader. Forexample, an electronic book reader 2700 includes two housings, a housing2701 and a housing 2703. The housing 2701 and the housing 2703 arecombined with the use of a hinge 2711 so that the electronic book reader2700 can be opened and closed using the hinge 2711 as an axis. With sucha structure, the electronic book reader 2700 can be handled like a paperbook.

A display portion 2705 and a photoelectric conversion device 2706 areincorporated in the housing 2701. A display portion 2707 and aphotoelectric conversion device 2708 are incorporated in the housing2703. The display portion 2705 and the display portion 2707 may displaya continuous image or different images. In the case where the displayportions display different images, for example, a display portion on theright (the display portion 2705 in FIG. 11) can display text and adisplay portion on the left (the display portion 2707 in FIG. 11) candisplay graphics.

FIG. 11 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. Pages can be turned with the operation keys 2723. Notethat a keyboard, a pointing device, and the like may be provided on thesame surface as the display portion of the housing. Moreover, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal connectable to an AC adapter or a variety of cables such as aUSB cable, or the like), a storage medium insertion portion, and thelike may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may have a functionof an electronic dictionary.

The electronic book reader 2700 may be configured to wirelessly transmitand receive data. Through wireless communication, desired book data orthe like can be purchased and downloaded From an electronic book server.

EXAMPLE 1

In this example, a microcrystalline semiconductor film having highcrystallinity, in which a space between mixed phase grains in a seedcrystal is filled, can be formed by forming a microcrystallinesemiconductor film in three steps: formation of a seed crystal under afirst condition, formation of a first microcrystalline semiconductorfilm under a second condition, and formation of a secondmicrocrystalline semiconductor film under a third condition as describedin Embodiment 1.

First, a method for forming a microcrystalline semiconductor film usingthe method described in Embodiment 1 will be described.

A silicon nitride film with a thickness of 300 nm was formed as aninsulating film over a glass substrate (EAGLE XG manufactured by CorningIncorporated), and the silicon nitride film was exposed to plasmagenerated using a mixed gas of hydrogen and oxygen. Then, a seed crystalwith a thickness of 5 nm was formed by a plasma CVD method over thesilicon nitride film, a first microcrystalline film with a thickness of25 nm was formed by a plasma CVD method over the silicon nitride filmand the seed crystal, and then a second microcrystalline semiconductorfilm with a thickness of 40 nm was formed by a plasma CVD method overthe first microcrystalline semiconductor film.

The silicon nitride film was formed by plasma discharge performed underthe following conditions: silane, hydrogen, nitrogen, and ammonia wereintroduced as source gas at flow rates of 15 sccm, 200 sccm, 180 sccm,and 500 sccm, respectively; the pressure in a process chamber was 100Pa; the RF power supply frequency was 13.56 MHz; and the power of the RFpower supply was 200 W. Note that in the deposition of the siliconnitride film, a parallel-plate plasma CVD apparatus was used, thetemperature of an upper electrode was 250° C., the temperature of alower electrode was 290° C., and the distance (the gap) between theupper electrode and the lower electrode was 30 mm.

The plasma treatment on the silicon nitride film was performed by plasmadischarge for three minutes under the following conditions: hydrogen andoxygen were introduced into a process chamber at flow rates of 800 sccmand 200 sccm respectively, the pressure in the process chamber was 1250Pa, and the power was 900 W. Note that for the plasma treatment, aparallel-plate plasma treatment apparatus was used, the temperature ofan upper electrode was 250° C., the temperature of a lower electrode was290° C., and the distance between the upper electrode and the lowerelectrode was 15 mm.

The seed crystal was formed by plasma discharge performed under thefollowing conditions: silane, hydrogen, and argon were introduced assource gas at flow rates of 3 sccm, 750 sccm, and 750 sccm,respectively; the pressure in a process chamber was 1250 Pa; the RFpower supply frequency was 13.56 MHz; and the power of the RF powersupply was 90 W. Note that in the deposition of the seed crystal, aparallel-plate plasma CVD apparatus was used, the temperature of anupper electrode was 250° C., the temperature of a lower electrode was290° C., and the distance between the upper electrode and the lowerelectrode was 15 mm.

The first microcrystalline semiconductor film was formed by plasmadischarge performed under the following conditions: silane, hydrogen,and argon were introduced as source gas at flow rates of 2 sccm, 1500sccm, and 1500 sccm, respectively; the pressure in a process chamber was10000 Pa; the RF power supply frequency was 13.56 MHz; and the power ofthe RF power supply was 350 W. Note that in the deposition of the firstmicrocrystalline semiconductor film, a parallel-plate plasma CVDapparatus was used, the temperature of an upper electrode was 250° C.,the temperature of a lower electrode was 290° C., and the distancebetween the upper electrode and the lower electrode was 7 mm.

The deposition conditions other than the flow rate of silane for thesecond microcrystalline semiconductor film are similar to those for thefirst microcrystalline semiconductor film. A first period in whichsilane flows at a flow rate of 1 sccm for 10 seconds and a second periodin which silane flows at a flow rate of 0.1 sccm were repeated. The timefor which silane flows at a flow rate of 1 sccm is referred to as “Hightime”, and the time for which silane flows at a now rate of 0.1 sccm isreferred to as “Low time”.

Second microcrystalline semiconductor films were deposited under theconditions where the time for which silane flows at a flow rate of 0.1sccm (Low time) is 5 seconds, 10 seconds, 15 seconds, 20 seconds, 25seconds, 30 seconds, 45 seconds, and 60 seconds. Then, the crystallinityof the microcrystalline semiconductor films was analyzed by Ramanspectroscopy. In the graph of FIG. 12A, rhombuses represent thecrystalline/amorphous peak intensity ratio (Ic/Ia) at three points inthe respective microcrystalline semiconductor films, and rectanglesrepresent the full width at half maximum (FWHM) of the peak at 520 cm⁻¹in respective Raman spectra. In FIG. 12A, the left y-axis shows theIc/Ia against Low time on the horizontal axis, and the right y-axisshows the FWHM against Low time on the horizontal axis.

As Low time becomes longer with respect to High time, the value of Ic/Iabecomes larger and the value of FWHM becomes smaller. This shows thatwhen the time for which silane flows at a low flow rate (Low time)becomes longer with respect to the time for which silane flows at a highflow rate (High time), the crystallinity of the microcrystallinesemiconductor film is increased.

Next, microcrystalline semiconductor films were formed by setting Hightime at 10 seconds and Low time at 15 seconds, 30 seconds, and 60seconds in the deposition of the second microcrystalline semiconductorfilm, and a microcrystalline semiconductor film was formed as acomparative example by setting High time at 10 seconds and Low time at 5seconds. Then, the size of a crystallite in a mixed phase grain in theabove microcrystalline semiconductor films was evaluated by in-planeX-ray diffraction (in-plane XRD). The film density of the respectivemicrocrystalline semiconductor films was analyzed by X-ray reflectometry(XRR). The results are shown in FIG. 12B. In FIG. 12B, the left y-axisshows the size of a crystallite against Low time on the horizontal axis.Bars individually indicate the size of crystallites having (111)orientation, (220) orientation, and (311) orientation. Further, in thebar chart, the bar with vertical hatching indicates the size of acrystallite having (111) orientation, the bar with hatching that isoblique to the upper right indicates the size of a crystallite having(220) orientation, and the bar with hatching that is oblique to theupper left indicates the size of a crystallite having (311) orientation.

The right y-axis and rhombuses show the film density against Low time onthe horizontal axis. Note that in the XRR analysis, fitting wasperformed on four-layer structure models: an interface layer with a basefilm, a main layer, a surface roughness layer (1), and a surfaceroughness layer (2), into which the microcrystalline semiconductor filmwas divided from the silicon nitride film side. The average density ofthe four layers is largely affected by the surface roughness layer (1)and the surface roughness layer (2). Thus, in order to separatelyevaluate only an increase and decrease of a space between mixed phasegrains in the microcrystalline semiconductor films, the value of themain layer was compared.

In FIG. 12B, the film density of the microcrystalline semiconductorfilms in which the second microcrystalline semiconductor film isdeposited under a condition where Low time is 5 seconds or more is 2.30g/cm³ or higher. Since the film density of single crystal silicon is2.33 g/cm³, it can be said that there is extremely little space betweenmixed phase grains in any of the microcrystalline semiconductor films.Further, the Microcrystalline semiconductor films in which the secondmicrocrystalline semiconductor film is deposited under a condition whereLow time is longer than High time, include a crystallite having a sizeof 13 nm or more.

Next, SEM photographs of a microcrystalline semiconductor film formedunder a condition where Low time is 60 seconds (referred to as Sample 1)and a microcrystalline semiconductor film formed under a condition whereLow time is 5 seconds (referred to as Sample 2), which were observedwith a scanning electron microscope, are shown in FIGS. 13A and 13B andFIGS. 14A and 14B. FIG. 13A and FIG. 14A are photographs of planes(magnified 0.2 million times), and FIG. 13B and FIG. 14B are photographsof cross sections (magnified 0.2 million times).

In the microcrystalline semiconductor film in FIG. 13A, more angularmixed phase grains than those in FIG. 14A are observed. In addition, aregion projecting from the film surface can be seen prominently in themicrocrystalline semiconductor film in FIG. 13B, as compared to themicrocrystalline semiconductor film in FIG. 14B.

Next, TEM photographs of the formed microcrystalline semiconductor filmsof Sample 1 and Sample 2, which were observed with a high resolutiontransmission electron microscope, are shown in FIGS. 15A and 15B andFIGS. 16A. and 16B. FIGS. 15A and 15B and FIGS. 16A and 16B arephotographs of cross sections of the respective samples; FIG. 15A andFIG. 16A are photographs with a magnification of 0.5 million times andFIG. 15B and FIG. 16B are photographs with a magnification of 2 milliontimes.

Mixed phase grains partly projecting from the surface are included morein the microcrystalline semiconductor film in FIG. 15A than in themicrocrystalline semiconductor film in FIG. 16A. As shown in FIG. 15B,the projecting mixed phase grains have substantially the same contrastand have an orientation plane on the surface. Further, lines can be seenin the mixed phase grain, which shows that a twin crystal or a stackingfault is included therein.

Accordingly, by using the formation method described in Embodiment 1, amicrocrystalline semiconductor film which has high crystallinity and inwhich mixed phase grains are densely packed with extremely little spacebetween adjacent mixed phase grains can be formed.

Comparative Example

Here, in the formation method of the second microcrystallinesemiconductor film described in Example 1, the flow rates of silane andhydrogen were set constant without employing cycle flow; thus, amicrocrystalline semiconductor film was formed. This microcrystallinesemiconductor film will be described with reference to FIG. 17.

A silicon nitride film with a thickness of 300 nm was formed as aninsulating film over a glass substrate (EAGLE XG manufactured by CorningIncorporated), and the silicon nitride film was exposed to plasmagenerated in a dinitrogen monoxide atmosphere. Then, a seed crystal witha thickness of 5 nm was formed by a plasma CVD method over the siliconnitride film, a first microcrystalline semiconductor film with athickness of 25 nm was formed by a plasma CVD method over the siliconnitride film and the seed crystal, and then a second microcrystallinesemiconductor film was formed by a plasma CVD method over the firstmicrocrystalline semiconductor film.

The silicon nitride film was formed at a flow rate of the source gas anda power of the plasma CVD apparatus which were similar to those inExample 1. Note that in the parallel-plate plasma CVD apparatus, thetemperature of the upper electrode was 200° C., the temperature of thelower electrode was 300° C., and the distance between the upperelectrode and the lower electrode was 26 mm.

The plasma treatment on the silicon nitride film was performed by plasmadischarge for three minutes under the following conditions: dinitrogenmonoxide was introduced into a process chamber at a flow rate of 400sccm, the pressure in the process chamber was 60 Pa, and the power was900 W. Note that for the plasma treatment, a parallel-plate plasmatreatment apparatus was used, the temperature of an upper electrode was200° C., the temperature of a lower electrode was 300° C., and thedistance between the upper electrode and the lower electrode was 30 mm.

In the deposition conditions of the seed crystal, the same flow rate ofthe source gas as Example 1 was employed. The power of the RF powersupply was set to 150 W. in addition, in the parallel-plate plasma CVDapparatus, the temperature of the upper electrode was not controlled bya heater, the temperature of the lower electrode was set to 300° C., andthe distance between the upper electrode and the lower electrode was setto 7 mm.

The first microcrystalline semiconductor film was formed by plasmadischarge performed under the following conditions: silane and hydrogenwere introduced as source gas at flow rates of 2 scan and 3000 sccmrespectively; the pressure in a process chamber was 10000 Pa; the RFpower supply frequency was 13.56 MHz; and the power of the RF powersupply was 700 W. Note that in the deposition of the firstmicrocrystalline semiconductor film, the parallel-plate plasma CVDapparatus was used, the temperature of the upper electrode was notcontrolled by a heater, the temperature of the lower electrode was 300°C., and the distance between the upper electrode and the lower electrodewas 7 mm.

In forming the second microcrystalline semiconductor film, thedeposition conditions other than the flow rates of silane and hydrogenwere similar to those for the first microcrystalline semiconductor film.Here, plasma discharge was performed for 2400 seconds at a flow rate ofsilane of 0.1 sccm and a flow rate of hydrogen of 300 sccm.

A SEM photograph of the microcrystalline semiconductor film formed inthe above-described process, which was observed with a scanning electronmicroscope, is shown in FIG. 17. FIG. 17 is the photograph of a plane(magnified 0.2 million times). As can be noticed from FIG. 17, a mixedphase grain having an orientation plane cannot be observed in themicrocrystalline semiconductor film formed as the comparative example.Further, the average crystalline/amorphous peak intensity ratio (Ic/Ia)in three points in the microcrystalline semiconductor film was 10.8, andthe average full width at half maximum (FWHM) of the peaks at 520 cm⁻¹in Raman spectra was 11.8. From these results, it is found that thecrystallinity of the microcrystalline semiconductor film is not improvedif cycle flow is not performed in the step for forming the secondmicrocrystalline semiconductor film. Moreover, the thickness of themicrocrystalline semiconductor. film formed as the comparative examplewas 30.8 nm, which means microcrystalline semiconductor was hardlydeposited in the step for forming the second microcrystallinesemiconductor film.

From the above, it can be found that a microcrystalline semiconductorfilm having high crystallinity can be formed by employing cycle flow inthe step for forming the second microcrystalline semiconductor film.

This application is based on Japanese Patent Application serial no.2011-012496 filed with Japan Patent Office on Jan. 25, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a microcrystallinesemiconductor film, comprising the steps of: forming a seed crystal overan insulating film by a plasma CVD method under a first condition;forming a first microcrystalline semiconductor film on the seed crystalby a plasma CVD method under a second condition; and forming a secondmicrocrystalline semiconductor film on the first microcrystallinesemiconductor film by a plasma CVD method under a third condition,wherein the first condition is a condition that a deposition gascontaining silicon is diluted by setting a flow rate of hydrogen to morethan or equal to 50 times and less than or equal to 1000 times that ofthe deposition gas, and a pressure in a process chamber is higher thanor equal to 67 Pa and lower than or equal to 50000 Pa, wherein thesecond condition is a condition that a deposition gas containing siliconis diluted by setting a flow rate of hydrogen to more than or equal to100 times and less than or equal to 2000 times that of the depositiongas, and a pressure in the process chamber is higher than or equal to1333 Pa and lower than or equal to 50000 Pa, wherein the third conditionis a condition that a pressure in the process chamber is higher than orequal to 1333 Pa and lower than or equal to 50000 Pa, and a first periodin which microcrystalline semiconductor is deposited and a second periodwhich is longer than the first period and in which an amorphoussemiconductor region formed in the first period is selectively etchedare alternately performed, and wherein the microcrystallinesemiconductor film is formed by the seed crystal, the firstmicrocrystalline semiconductor film, and the second microcrystallinesemiconductor film.
 2. The method for manufacturing a microcrystallinesemiconductor film according to claim 1, wherein the seed crystalcomprises a mixed phase grain including both an amorphous region and acrystalline region.
 3. The method for manufacturing a microcrystallinesemiconductor film according to claim 1, wherein a flow rate of hydrogenis fixed and a flow rate of the deposition gas containing silicon isincreased and decreased in the third condition.
 4. The method formanufacturing a microcrystalline semiconductor film according to claim1, wherein the microcrystalline semiconductor film comprises a crystalgrain, wherein the crystal grain has an orientation plane, and whereinthe crystal grain comprises a crystallite having a size of 13 nm or moreon a top surface of the microcrystalline semiconductor film.
 5. Themethod for manufacturing a microcrystalline semiconductor film accordingto claim 4, wherein the crystal grain projects from the top surface ofthe microcrystalline semiconductor film.
 6. The method for manufacturinga microcrystalline semiconductor film according to claim 1, wherein themicrocrystalline semiconductor film has a film density of higher than orequal to 2.25 g/cm³ and lower than or equal to 2.35 g/cm³.
 7. The methodfor manufacturing a microcrystalline semiconductor film according toclaim 1, wherein a rare gas is contained in the deposition gas.
 8. Amethod for manufacturing a semiconductor device by using the method formanufacturing a microcrystalline semiconductor film according toclaim
 1. 9. A method for manufacturing a semiconductor device,comprising the steps of: forming a gate electrode over a substrate;forming a gate insulating film over the gate electrode; forming a seedcrystal over the gate insulating film under a first condition; forming afirst microcrystalline semiconductor film on the seed crystal under asecond condition; forming a second microcrystalline semiconductor filmon the first microcrystalline semiconductor film under a thirdcondition; forming a semiconductor film comprising a microcrystallinesemiconductor region and an amorphous semiconductor region over thesecond microcrystalline semiconductor film; forming a first impuritysemiconductor film over the semiconductor film; forming an island-shapedsecond impurity semiconductor film by etching part of the first impuritysemiconductor film; forming an island-shaped first semiconductor stackedbody by etching part of the seed crystal, part of the firstmicrocrystalline semiconductor film, part of the second microcrystallinesemiconductor film, and part of the semiconductor film; forming a wiringover the island-shaped second impurity semiconductor film; and forming apair of impurity semiconductor films by etching the island-shaped secondimpurity semiconductor film, wherein the first condition is a conditionthat a deposition gas containing silicon is diluted by setting a flowrate of hydrogen to more than or equal to 50 times and less than orequal to 1000 times that of the deposition gas, and a pressure in aprocess chamber is higher than or equal to 67 Pa and lower than or equalto 50000 Pa, wherein the second condition is a condition that adeposition gas containing silicon is diluted by setting a flow rate ofhydrogen to more than or equal to 100 times and less than or equal to2000 times that of the deposition gas, and a pressure in the processchamber is higher than or equal to 1333 Pa and lower than or equal to50000 Pa, and wherein the third condition is a condition that a pressurein the process chamber is higher than or equal to 1333 Pa and lower thanor equal to 50000 Pa, and a first period in which microcrystallinesemiconductor is deposited and a second period which is longer than thefirst period and in which an amorphous semiconductor region formed inthe first period is selectively etched are alternately performed. 10.The method for manufacturing a semiconductor device according to claim9, further comprising the step of forming an insulating region on a sidesurface of the island-shaped first semiconductor stacked body byexposing the side surface of the island-shaped first semiconductorstacked body to plasma, after the step of forming the island-shapedfirst semiconductor stacked body and before the step of forming thewiring.
 11. The method for manufacturing a semiconductor deviceaccording to claim 9, wherein the seed crystal comprises a mixed phasegrain including both an amorphous region and a crystalline region. 12.The method for manufacturing a semiconductor device according to claim9, wherein a flow rate of hydrogen is fixed and a flow rate of thedeposition gas containing silicon is increased and decreased in thethird condition.
 13. The method for manufacturing a semiconductor deviceaccording to claim 9, wherein a rare gas is contained in the depositiongas.
 14. The method for manufacturing a semiconductor device accordingto claim 9, further comprising the steps of: forming a secondsemiconductor stacked body in which a microcrystalline semiconductorregion and a pair of amorphous semiconductor regions are stacked, byetching part of the island-shaped first semiconductor stacked body;forming an insulating film over the wiring, the pair of impuritysemiconductor films, the second semiconductor stacked body, and the gateinsulating film; and forming a back gate electrode and a pixel electrodeover the insulating film.
 15. The method for manufacturing asemiconductor device according to claim 14, wherein the gate electrodeand the back gate electrode are provided substantially parallel to eachother.
 16. The method for manufacturing a semiconductor device accordingto claim 14, wherein the gate electrode and the back gate electrode areelectrically connected to each other.
 17. The method for manufacturing asemiconductor device according to claim 14, wherein the back gateelectrode is in a floating state.
 18. The method for manufacturing asemiconductor device according to claim 14, wherein the back gateelectrode and the pixel electrode are formed at a same time.
 19. Themethod for manufacturing a semiconductor device according to claim 9,wherein the second microcrystalline semiconductor film is formed tocomprise a crystal grain, wherein the crystal grain comprises acrystallite having a size of 13 nm or more on a top surface of thesecond microcrystalline semiconductor film.
 20. The method formanufacturing a semiconductor device according to claim 19, wherein thecrystal grain projects from the top surface of the secondmicrocrystalline semiconductor film.
 21. The method for manufacturing asemiconductor device according to claim 9, a microcrystallinesemiconductor film formed by the seed crystal, the firstmicrocrystalline semiconductor film, and the second microcrystallinesemiconductor film has a film density of higher than or equal to 2.25g/cm³ and lower than or equal to 2.35 g/cm³.